Acrylamide polymers and use thereof

ABSTRACT

Provided by this invention are an acrylamide polymer which has a weight-average molecular weight (a) of from 1,500,000 to 10,000,000 and a weight-average inertial radius (b) of from 30 to 150 nm, the ratio (b)/(a) of the weight-average inertial radius (b) to the weight-average molecular weight (a) being 0.00004 or smaller; and an aqueous solution of an acrylamide polymer which solution has a polymer concentration ranging from 22 to 60%, a Brookfield viscosity at 25° C. of 50,000 cps or smaller and a weight-average molecular weight of from 500,000 to 10,000,000. They are useful as paper strength agents.

This application is a Continuation of application Ser. No. 08/753,434,filed on Nov. 25, 1996, now abandoned, which is a Division ofapplication Ser. No. 08/363,268, filed Dec. 23, 1994, now U.S. Pat. No.5,627,249.

BACKGROUND OF THE INVENTION

a) Field of the Invention

This inventions relates to acrylamide polymers and aqueous solutionsthereof, which are useful in papermaking industry, the field of effluenttreatment, the field of civil engineering and construction, and thelike. Specifically, the present invention is concerned with acrylamidepolymers having a controlled branched and crosslinked structure. Theacrylamide polymers according to the present invention are particularlyuseful in the field of papermaking industry as paper strength agentsexcellent in paper reinforcing effects.

b) Description of the Related Art

Acrylamide polymers have been used for various applications to date.Specifically, paper strength agents, drainage agents, retention aids,coagulants, secondary oil recovering agents, dispersants, etc. In theseapplications, various acrylamide polymers whose weight average molecularweights range from about 100,000 to somewhat greater than 20,000,000 arechosen depending on the application purposes, and those having a linearmolecular structure have been used primarily.

In recent years, it has been attempted to improve the performance ofacrylamide polymers in the field of papermaking industry, such as paperstrength agents, by allowing them to have a partially-crosslinkedstructure and a partially-branched structure so that their molecularweights can be increased while suppressing a viscosity increase in theirsolutions.

There is, for example, a technique in which a vinyl monomer having twoor more double bonds in a molecule is used as a crosslinking agent.Specific known examples of the vinyl monomer include divinyl monomerssuch as methylenebisacrylamide, methylenebismethacrylamide, ethyleneglycol diacrylate and ethylene glycol dimethacrylate; and trifunctionalmonomers such as 1,3,5-triacryloylhexahydro-S-triazine and triallylisocyanurate. The molecular weight increasing technique, which reliesupon these vinyl monomers alone, are however still considered to beinsufficient for paper strength agents, because it results in a branchedand crosslinked structure substantially lacking uniformity and moreover,increased gelling effects or the like are observed when polyfunctionalmonomers are used.

It has also been attempted to produce an acrylamide polymer having abranched and crosslinked structure by using a specific compound incombination with such a crosslinking agent. Known combinations include,for example, the combinations of crosslinking agents and primary aminesor OH-containing monomers, the combinations of crosslinking agents andspecific hydrophobic monomers, and the combinations of crosslinkingagents and itaconic acid derivatives. Although it is said that thesecombinations can all achieve an increase in molecular weight whilemaintaining the viscosity low but in view of the structures of theresulting polymers, they are not sufficient as paper strength agents andtheir effects are still insufficient.

To produce acrylamide polymers having a branched and crosslinkedstructure, attempts have also been made using means other than theabove-described branching and crosslinking method which relies uponcopolymerization making use of a polyfunctional vinyl monomer. As oneexample of this approach, a papermaking additive has been proposed,which uses a reaction between a specific N-substituted acrylamidederivative such as N,N-dimethylacrylamide and a persulfate- orperoxide-base catalyst. This method is said to permit an increase inmolecular weight while maintaining the viscosity low but, because ofoccurrence of polymer degradations, the resulting polymer is notsufficient in view of its structure and is not considered to exhibitsufficient effects as a paper strength agent.

The conventional acrylamide polymers produced by these conventionaltechniques are generally used as aqueous solutions for theabove-described applications. Processes have been proposed for theproduction of an acrylamide polymer which has a molecular weight ofabout 3,000,000 or so and can be formed into an 15% aqueous solution. Noacrylamide polymer solution having a high concentration is however knownat all. The concentration is as low as 15% to 21% at most, because alinear acrylamide polymer which may be formed into an aqueous solutionhaving a concentration higher than the above level does not have such ahigh molecular weight as desired or the solution so formed has a veryhigh viscosity. In addition, any attempt to introduce a branched andcrosslinked structure into an acrylamide polymer causes a crosslinkingreaction to locally proceed to a considerable extent so that, coupledwith the high concentration, water-insoluble gel is formed or theresulting mixture is gelated entirely and the resulting acrylamidepolymer cannot be obtained in the form of an aqueous solution. Theacrylamide polymer is therefore not usable as a paper strength agent.

An application for patent was filed in Japan on a surface strength agentcomposed of an aqueous solution of a polyacrylamide polymer, which has aviscosity of 500-5,000 cps and a concentration of 10-30%. Theapplication was published under Japanese Patent Laid-Open No.279491/1991. Although a molecular weight range of 100,000 to 3,000,000is referred to in its specification, there is no specific disclosure asto the possibility that the process disclosed therein may achieve amolecular weight increase to a level as high as 500,000 or greater interms of absolute molecular weight while permitting a concentration of22% or higher.

Despite a merit available in shipping cost from a higher polymerconcentration, no aqueous solution containing a high molecular weightacrylamide polymer at a concentration of 22% or higher was known at allto date for the reasons described above.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an aqueous solution ofan acrylamide polymer having novel physical properties unknown in theconventional art, specifically, an aqueous solution of an acrylamidepolymer, said solution having a relatively low viscosity at a highconcentration despite a high molecular weight of the acrylamide polymer,and also a process for the preparation of the aqueous solution.

Another object of the present invention is to provide an acrylamidepolymer having a novel structure unknown in the above-describedconventional art.

A further object of the present invention is to provide a paper strengthagent comprising said acrylamide polymer.

With the foregoing circumstances in view, the present inventors haveproceeded with an extensive investigation on the polymerization ofacrylamide polymers. This investigation has now resulted in theprovision of an aqueous polymer solution which still has a low viscosityalthough the solution and the polymer have such a high polymerconcentration and high molecular weight as unavailable to date.

In one aspect of the present invention, there is thus provided anacrylamide polymer having (a) a weight-average molecular weight of from1,500,000 to 10,000,000 and (b) a weight-average root mean square radiusof from 30 to 150 nm, the ratio (b)/(a) of the weight-average root meansquare radius to the weight-average molecular weight being 0.00004 orsmaller.

The acrylamide polymer can preferably have (c) a number-averagemolecular weight of from 400,000 to 5,000,000, the ratio (a)/(c) of theweight-average molecular weight to the number-average molecular weightbeing 6 or smaller.

In another aspect of the present invention, there is also provided anaqueous solution of an acrylamide polymer, which has a polymerconcentration ranging from 22 to 60%, a Brookfield viscosity at 25° C.of 50,000 cps or smaller and a weight-average molecular weight of from500,000 to 10,000,000.

In a further aspect of the present invention, there is also provided asa preferred embodiment a paper strength agent comprising a water-solublepolymer obtained by polymerizing the following components:

(a) from 94 to 99.98 mole % of (meth)acrylamide;

(b) from 0.01 to 1 mole % of a crosslinking monomer; and

(c) from 0.01 to 5 mole % of one or more of vinyl compounds and/or saltsthereof, said vinyl compounds being represented by the following formula(1): ##STR1## wherein R represents a hydrogen atom or a C₁₋₃ lower alkylgroup and n stands for an integer of from 1 to 4.

In necessary, 0.01-20 mole % of the (meth)acrylamide (a) can be replacedby the same molar amount of one or more of vinyl compounds and/or saltsthereof copolymerizable with the crosslinking monomer.

DETAILED DESCRIPTION OF THE INVENTION

The term "acrylamide polymer" as used herein means a polymer formed fromacrylamide and/or methacrylamide or an acrylamide copolymer containingacrylamide and/or methacrylamide as a predominant component among allmonomers making up the copolymer. When employed as a paper strengthagent, however, the acrylamide polymer preferably contains acrylamideand/or methacrylamide in a proportion of 50 mole % or more, preferably74-99.97 mole %, and especially 94-99.98 mole %.

The term "weight average molecular weight (a)" as used herein can bedetermined by the static light scattering method. Specifically, itsvalue can be obtained by using a multi-angle light scattering detectorand then preparing a Zimm plot or the like. As an alternative, it canalso be obtained by preparing a Debye plot in accordance with theGPC-MALLS method while using a gel permeation chromatograph (GPC)equipped with a multi-angle laser light scattering detector connectedthereto.

Further, the weight average root mean square radius (b) is measured bythe GPC-MALLS method referred to above.

In general, the following fundamental formula of light scattering isused for the measurement of a molecular weight by the light scatteringmethod.

    Kc/R(θ)=1/MwP(θ)+2A.sub.2 c+. . .

where

R(θ): The excess Rayleigh ratio of light scattered at the angle (θ);

c: Concentration of the sample;

Mw: Weight average molecular weight;

A₂ : Second virial coefficient;

K: Optical parameter; and

P(θ): Normalized intensity distribution function.

THe term "weight average molecular weight (a)" as used herein means avalue obtained by ignoring the second and higher terms, which representthe second virial coefficient, like the GPC-LALLS method which isconducted using a GPC and a low-angle laser light scattering detectorconnected thereto.

Further, the term "weight average root mean square radius (b)" as usedherein is generally expressed by <S² >^(1/2).

To ascertain the average crosslinking degree of a polymer, it isimportant to find out the relationship between its weight averagemolecular weight and its molecular size (e.g., root mean square radiusor the like). It is generally known that, where polymers have the samecomposition and the same molecular weight, the molecular size becomessmaller as the crosslinking degree becomes greater. Accordingly, whenthe weight average root mean square radius (b) and the weight averagemolecular weight (a) are specified, their ratio (b)/(a) can be used asan index showing an average crosslinking degree.

The weight average molecular weight (a) and weight average root meansquare radius (b) of the acrylamide polymer according to the presentinvention are 1,500,000-10,000,000 and 30-150 nm, respectively, and the(b)/(a) ratio is 0.00004 or smaller. In view of the preference toward auniform branched and crosslinked structure, the preferred weight averagemolecular weight (a) and weight average inertial radius (b) can be2,000,000-8,000,000 and 40-120 nm, respectively, and the preferred(b)/(a) ratio can be 0.000035 or smaller. A more preferable (b)/(a)ratio is 0.00003 or smaller.

The preferred number average molecular weight (c) of the acrylamidepolymer according to this invention can be 400,000-5,000,000, and thepreferred ratio of its weight average molecular weight (a) to (c), thatis, the preferred (a)/(c) ratio can be 6 or smaller. In view of thepreference toward a uniform branched and crosslinked structure, the morepreferred number average molecular weight (c) can be 500,000-3,000,000,and the more preferred (a)/(c) ratio can be 4 or smaller. Incidentally,the number average molecular weight (c) described above means anabsolute number average molecular weight, which can be measured by theGPC-MALLS method. The ratio of the weight average molecular weight (a)to the number average molecular weight (c), that is, the (a)/(c) ratioshows the molecular weight distribution of the polymer.

In the acrylamide polymer according to the present invention, moleculeshaving a molecular weight of 1,000,000 or greater can preferably amountto 40 wt. % or more. In view of the preference toward a uniform branchedand crosslinked structure, such molecules can more preferably amount to50 wt. % or more, with 60 wt. % or more being particularly preferred.The term "molecular weight" as used here means an absolute molecularweight measured by the GPC-MALLS method and can be obtained from adistribution curve of absolute molecular weights by the GPC-MALLSmethod.

Values of the polymer, such as its molecular weight and root mean squareradius, by the above-described light scattering method can be measuredusing as a solvent (or an eluent) a 1/15N phosphate buffer (pH 7) whichcontains 1/10N of sodium nitrate.

In the acrylamide polymer according to the present invention, thehydrated diameter by the dynamic light scattering method can preferablybe 50-300 nm, with 70-300 nm being more preferred. The hydrated diameterby the dynamic light scattering method means a hydrodynamic diameteravailable in accordance with the Einstein-Stokes' equation and is avalue obtained by measuring the polymer at a 0.1% concentration, ascattering angle of 90° and 20° C. while using as a solvent (or aneluent) a 1/15N phosphate buffer (pH 7) containing 1/10N of sodiumnitrate. Described specifically, it can be measured by an instrumentsuch and a "Submicron Particle Analyzer Model N4" manufactured byCoulter Inc. In this instrument, analysis of data so obtained isperformed by SDP analysis making use of the program "CONTIN".

An aqueous solution of the acrylamide polymer according to the presentinvention can have a Brookfield viscosity of 20-10,000 cps/25° C. at 10%polymer concentration or a Brookfield viscosity of 100-30,000 cps/25° C.or preferably of 300-20,000 cps/25° C. at 15% polymer concentration.

Examples of the acrylamide polymer according to this invention includethose obtained by copolymerizing acrylamide or methacrylamide with oneor more of compounds or salts thereof, said compounds being representedby the following formula (1): ##STR2## wherein R represents a hydrogenatom or a lower alkyl group and n stands for an integer of from 1 to 8.

In the formula (1), the lower alkyl group is preferably a C₁₋₃ loweralkyl group, and specifically a methyl, ethyl, n-propyl or i-propylgroup. Examples of the salts include the salts of alkali metals such assodium and potassium, and the ammonium salt. Specific examples ofsulfonic acid compounds represented by the formula (1) includeallylsulfonic acid, sodium allylsulfonate, methallylsulfonic acid,sodium methallylsulfonate, and ammonium methallylsulfonate.

The amount of the sulfonic acid or the salt thereof can amount to0.005-30 mole % based on all the monomers making up the acrylamidepolymer. In view of the preference toward a uniform branched andcrosslinked structure, however, it can amount preferably to 0.01-20 mole%, more preferably 0.05-10 mole %. When employed as an internal paperstrength agent, the sulfonic acid or salt thereof can preferably amountto 0.01-5 mole %, most preferably 0.05-5 mole %. These compounds can beused either singly or in combination.

Incidentally, with respect to mathallylsulfonic acid and salts thereof,their effects as molecular weight modifiers upon polymerization ofacrylamide are disclosed in U.S. Pat. No. 4,451,628. The processdisclosed in this patent however merely indicates the possibility ofprovision of an acrylamide polymer of low molecular weight, and thepolymer so obtained is totally different from a polymer which isavailable in accordance with the present invention and has a controlled,branched and crosslinked structure.

Further, use of a crosslinkable monomer in addition to the compoundrepresented by the formula (1) makes it possible to more easily obtainthe acrylamide polymer of this invention.

Specific examples of such crosslinkable monomers include bifunctionalcrosslinkable monomers such as methylenebisacrylamide,methylenebismethacrylamide, ethylenebisacrylamide,ethylenebismethacrylamide, ethylene glycol diacrylate, ethylene glycoldimethacrylate, diethylene glycol diacrylate, diethylene glycoldimethacrylate, triethylene glycol diacrylate, triethylene glycoldimethacrylate, divinylbenzene and diallylacrylamide; and polyfunctionalcrosslinkable monomers such as 1,3,5-triacryloylhexahydro-S-triazine,triallyl isocyanurate, triacrylic pentaerythritol, trimethylolpropaneacrylate, and triacrylformal, diacryloylimide. Such a crosslinkablemonomer may preferably amount to 0.005-5 mole % of the total amount ofall the monomers making up the acrylamide polymer. In view of thepreference toward a uniform branched and crosslinked structure, 0.01-2mole % is more preferred, with 0.01-1 mole % being most preferred. Thesecompounds can be used either singly or in combination.

The acrylamide polymer according to this invention can also be producedby copolymerizing one or more various vinyl monomers in addition toacrylamide and/or methacrylamide. Examples of such vinyl polymersinclude ionic monomers, hydrophilic monomers and hydrophobic monomers.

Among ionic monomers, exemplary anionic monomers include unsaturatedcarboxylic acids such as acrylic acid, methacrylic acid, itaconic acid,maleic acid and fumaric acid, and salts thereof; and vinylsulfonic acid,styrenesulfonic acid and acrylamidomethylpropanesulfonic acid, and saltsthereof.

Illustrative examples of cationic monomers include amines such asN,N-dimethylaminoethyl methacrylate, N,N-diethylaminoethyl methacrylate,N,N-dimethylaminoethyl acrylate, N,N-dimethylaminopropyl methacrylamide,and N,N-dimethylaminopropylacrylamide; and salts thereof (includingquaternary salts).

Illustrative examples of hydrophilic monomers include acetoneacrylamide,N,N-dimethylacrylamide, N,N-dimethylmethacrylamide,N-ethylmethacrylamide, N-ethylacrylamide, N,N-diethylacrylamide,N-propylacrylamide, N-acryloylpyrrolidine, N-acryloylpiperidine,N-acryloylmorpholine, hydroxyethyl methacrylate, hydroxyethyl acrylate,hydroxypropyl methacrylate, hydroxypropyl acrylate, variousmethoxypolyethylene glycol (meth)acrylates, and N-vinyl-2-pyrrolidone.

Exemplary hydrophobic monomers include N-alkyl (meth)acrylamidederivatives such as N,N-di-n-propylacrylamide, N-n-butylacrylamide,N-n-hexylacrylamide, N-n-hexylmethacrylamide, N-n-octylacrylamide,N-n-octylmethacrylamide, N-tert-octylacrylamide, N-dodecylacrylamide,and N-n-dodecylmethacylamide; N-(ω-glycidoxyalkyl)(meth)acrylamidederivatives such as N,N-diglycidylacrylamide,N,N-diglycidylmethacrylamide, N-(4-glycidoxybutyl)acrylamide,N-(4-glycidoxybutyl)methacrylamide, N-(5-glycidoxypentyl)acrylamide, andN-(6-glycidoxyhexyl)acrylamide; (meth)acrylate derivatives such asmethyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate,lauryl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and glycidyl(meth)acrylate; olefins such as acrylonitrile, methacrylonitrile, vinylacetate, vinyl chloride, vinylidene chloride, ethylene, propylene, andbutene; styrene; α-methylstyrene; butadiene; and isoprene.

Such vinyl monomers to be copolymerized can be used generally in a rangeof 0-50 mole %, although this amount varies depending on the kind andcombination of the monomers and cannot be defined in a wholesale manner.When the acrylamide polymer is used as a paper strength agent, it isoptimal to use an anionic monomer preferably in an amount of 0-20 mole%, more preferably in an amount of 0.5-10 mole % and also a cationicmonomer preferably in an amount of 0-20 mole %, more preferably 5-10mole %.

As a polymerization process for the acrylamide polymer in the presentinvention, radical polymerization is preferred. As a polymerizationsolvent, a polar solvent such as water, an alcohol or dimethylformamideis preferred. Aqueous solution polymerization is however preferred whenthe acrylamide polymer is used as a paper strength agent. In the case ofaqueous water polymerization, an organic solvent such as an alcohol canbe used in combination to such an extent that the dispersibility wouldnot be impaired by deposition or precipitation of the polymer.

The polymerization of the acrylamide polymer in the present inventioncan-be conducted by such a batch process that all the monomers arecharged at once in a reaction vessel and are then polymerized. To obtainan aqueous solution having a high concentration of 22% or more, it ishowever more desired to conduct the polymerization by such a semi-batchprocess that the polymerization is conducted while adding dropwise aportion of or the entire portion of the monomers. This semi-batchpolymerization process makes it possible not only to facilitate removalof polymerization heat from a solution containing monomers at highconcentrations but also to control the molecular structure, for example,to facilitate the formation of the polymer into a uniform branched andcrosslinked structure.

No particular limitation is imposed on the polymerization concentration,which is defined by the concentration of monomers and a polymer at thetime of polymerization. The polymerization concentration is generally2-40 wt. %, preferably 5-40 wt. %, To form an aqueous solution having ahigh concentration of 22% or more, however, the polymerizationconcentration can preferably be set as will be described hereinafter.

In batchwise polymerization. the polymerization concentration isgenerally 22-40 wt. %. Although it is possible to conduct polymerizationat a concentration lower than 22 wt. % and to convert the resultantpolymerization reaction mixture into an aqueous polymer solution havinga concentration of 22% or higher by concentration, this isdisadvantageous from the standpoint of economy. In semi-batchpolymerization, the polymerization concentration in a reaction vessel towhich reactants are being added dropwise can be set at a desired levelby adjusting the initial monomer concentration in the reaction vesseland also the dropping rates of the monomers. The polymerizationconcentration at the time of completion of the dropwise addition ishowever approximately 20-60 wt. %. In this case, it is also possible,like the batchwise polymerization, to conduct polymerization at aconcentration lower than 22 wt. % and then to convert the resultantpolymerization reaction mixture into an aqueous polymer solution havinga concentration of 22% or higher by concentration. This however involvessuch a disadvantage as described above.

No particular limitation is imposed on the polymerization initiator. Awater-soluble polymerization initiator is preferred. The polymerizationinitiator can be added either at once or dropwise to the aqueoussolution of the monomers. Specific examples of the polymerizationinitiator include, as persulfate and peroxide types, ammoniumpersulfate, potassium persulfate, sodium persulfate, hydrogen peroxide,benzoyl peroxide and tert-butyl peroxide. It is preferred to use suchpolymerization initiators singly, but they can also be combined with areducing agent for use as redox polymerization catalysts. Illustrativeexamples of the reducing agent include sulfites; bisulfites; low-degreeionization salts such as iron, copper and cobalt salts; organic aminessuch as N,N,N',N'-tetramethylethylenediamine; and reducing sugars suchas aldose and ketose.

An azo compound is also one of most preferred initiators for use in thepresent invention. Usable exemplary azo compounds include2,2'-azobis-2-methylpropionamidine hydrochloride,2,2'-azobis-2,4-dimethylvaleronitrile,2,2'-azobis-N,N'-dimethyleneisobutylamidine hydrochloride,2,2'-azobis-2-methyl-N-(2-hydroxyethyl)-propionamide,2,2'-azobis-2-(2-imidazolin-2-yl)propane and salts thereof, and4,4'-azobis-4-cyanovaleric acid and salts thereof. Two or more of theabove-described polymerization initiators can be used in combination.The amount of the polymerization initiator is generally 0.001-5 wt. %based on the monomers.

The polymerization temperature generally ranges from 30° C. to 90° C.where a single polymerization initiator is used. The initiationtemperature in the case of a redox polymerization initiator is lower andis generally 5-50° C. It is unnecessary to maintain the temperature atthe same level during the polymerization. The polymerization temperaturecan be changed as needed as the polymerization proceeds. Since thetemperature generally tends to arise due to heat of polymerization whichis produced as the polymerization proceeds, it may become necessary tocool the polymerization vessel as needed. Although no particularlimitation is imposed on the internal atmosphere of the polymerizationvessel in this case, it is desired to purge the internal atmosphere withinert gas such as nitrogen gas if one wants to make the polymerizationproceed quicker. No particular limitation is imposed on thepolymerization time but, including the time of the dropwise addition insemi-batch polymerization, the polymerization time is generally 1-20hours. Although no particular limitation is imposed on thepolymerization pH, the polymerization can be conducted by adjusting thepH as needed. Examples of pH modifiers include alkalizing agents such assodium hydroxide, potassium hydroxide and ammonia; mineral acids such asphosphoric acid, sulfuric acid and hydrochloric acid; and organic acidssuch as formic acid and acetic acid.

The present invention can provide an aqueous solution of an acrylamidepolymer having a Brookfield viscosity at 25° C. of 50,000 cps or lowerand a weight average molecular weight of 500,000-10,000,000 whilepermitting a concentration in a range as high as 22-60%, notably 30-60%.From the standpoint of uniformity in the branched and crosslinkedstructure, the preferred concentration and molecular weight are, forexample, in their respective ranges not inconvenient for the applicationof the aqueous solution as a paper strength agent, generally500,000-8,000,000 at 25% concentration, 500,000-6,000,000 at 30%concentration, 500,000-4,000,000 at 40% concentration, and500,000-3,000,000 at 50% concentration.

Incidentally, the polymer concentration of an aqueous polymer solutioncan be determined by measuring its oven-dry polymer concentration.Illustrative measuring methods include the hot-air drying method and theket method.

The polymers and aqueous solutions so obtained can exhibit variousexcellent effects as paper strength agents. Use of the acrylamidepolymers according to the present invention as paper strength agentswill be described in further detail.

Paper strength agents can be divided into two groups, one being used byadding the same into a pulp slurry and the other being employed byeither coating or impregnating a paper sheet with the same subsequent toremoval of water from the paper sheet by a wire. The acrylamide polymersaccording to the present invention can be used in either ways.

Each paper strength agent according to the present invention can be usedgenerally in a manner to be described hereinafter. Into a pulp slurry,it is added optionally together with a fixing agent such as aluminumsulfate. In this case, the paper strength agent is used approximately inan amount of 0.05-3% based on the solid content of the pulp slurry. Whenit is used as an external addition type, it is used in a similar amountby sizepress coating or calender coating.

When an aqueous acrylamide polymer solution whose concentration is 22%or higher is used as a paper strength agent, the acrylamide polymerpreferably has a weight average molecular weight of 500,000-10,000,000.A higher polymer concentration is obviously preferred from theeconomical standpoint too, but the viscosity is 50,000 cps or lower andfrom the standpoint of handling or workability during shipping or use,the preferred viscosity is 30,000 cps or lower, with a range not higherthan 20,000 being more preferred.

Incidentally, the aqueous polymer solution is used by diluting it asneeded.

It is not certain at this moment for what reasons the acrylamidepolymers having such novel structure and physical properties and theiraqueous solutions have been obtained by the process of the presentinvention. It is gathered that in the course of the polymerization ofthe compound represented by the formula (1) or (2), the resultantpolymer would undergo a specific reaction with radicals in anotherpolymer or with pendant double bonds derived from the crosslinkablemonomer to permit an efficient progress of branching and crosslinking,thereby making it possible to obtain an acrylamide polymer having astill more uniform branched and crosslinked structure compare with theacrylamide polymers known to date. It is also estimated that, owing tothe uniform branched and crosslinked structure of the polymer, thenumber of points of bonding between pulp fibers would have beenindirectly increased and the polymer would hence exhibit variousperformances as an excellent paper strength agent.

The present invention will hereinafter be described in further detail byExamples. It should however be borne in mind that this invention is byno means limited to or by the examples. Incidentally, all designationsof "%" mean wt. % unless otherwise specifically designated.

Measurements by GPC-MALLS in the examples were conducted under thefollowing conditions:

GPC apparatus: "SYSTEM 11" (trade name), manufactured by Showa DenkoK.K.

Column: "SHODEX SB 80M" (trade name)

Eluent: N/15 phosphate buffer solution (pH 7) containing N/10 sodiumnitrate

Flow rate: 1.0 ml/min

Detector: "DAWN" (trade name), a multi-angle laser light scatteringdetector manufactured by Wyatt Technology.

EXAMPLE 1

In a five-necked flask (hereinafter called "reaction vessel") equippedwith a stirrer, a reflux condenser, a thermometer, a nitrogen inlet tubeand a dropping funnel, 500 g of purified water were charged. Whilenitrogen gas was blown into the flask, the internal temperature wasadjusted to 80° C.

On the other hand, a mixed solution of 0.308 g of methylenebisacrylamide and 0.632 g of sodium methallylsulfonate in 354.3 g of a40% aqueous solution of acrylamide and 60 g of an aqueous solution inwhich 0.18 g of 4,4'-azobis-4-cyanovaleric acid having a purity of 84%had been dissolved were prepared, respectively. They were added dropwiseto the reaction vessel at constant rates over 130 minutes, respectively.In the course of the dropwise addition, the internal temperature of thereaction vessel was maintained at 80° C. After the completion of thedropwise addition, polymerization was continued for 3 hours at 80° C.Water was then added to the reaction vessel and the reaction vessel wasthen cooled to terminate the reaction, thereby obtaining an aqueoussolution of an acrylamide polymer having a Brookfield viscosity at 25°C. of 18,000 cps when the solution had a nonvolatile content of 15% anda Brookfield viscosity at 25° C. of 1,780 cps when the solution had anonvolatile content of 10%. The polymer so obtained is designated as"A-1". As a result of measurement of a weight-average molecular weight(a) and a weight-average root mean square radius (b) of "A-1" by theGPC-MALLS method, they were found to be 3,050,000 and 73.8 nm,respectively. The ratio of (b) to (a) was 0.0000242. In addition, it wasfound according to the above method that its absolute-number-averagemolecular weight (c) was 1,250,000, thereby giving an (a)/(c) ratio of2.44. Further, when the weight percentage (d) of molecular weights of1,000,000 and higher was determined from a molecular weight distributioncurve, it was found to be 73.5%. A-hydrated radius (e) was also measuredusing an N-4 type submicron particle analyzer manufactured by CoulterInc. As a result, it was found to be 120 nm.

EXAMPLE 2

In the reaction vessel, 560 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

In 354.1 g of a 40% aqueous solution of acrylamide, 1.23 g of methylenebisacrylamide, 0.79 g of sodium methallylsulfonate and 0.9 g of4,4'-azobis-4-cyanovaleric acid of 84% purity were mixed and dissolvedto prepare a mixed monomer-initiator solution.

The mixed monomer-initiator solution so obtained was added dropwise tothe reaction vessel at a constant rate over 150 minutes. In the courseof the dropwise addition, the internal temperature of the reactionvessel was maintained at 80° C. The temperature of the mixedmonomer-initiator solution was, on the other hand, maintained at 20° C.or lower so that the polymerization did not occur before the dropwiseaddition.

After the completion of the dropwise addition, polymerization wascontinued for 3 hours at 80° C. Water was then added to the reactionvessel and the reaction vessel was cooled to terminate the reaction,thereby obtaining an aqueous solution of an acrylamide polymer having aBrookfield viscosity at 25° C. of 2,170 cps when the solution had anonvolatile content of 15% and having a Brookfield viscosity at 25° C.of 330 cps when the solution had a nonvolatile content of 10%. Thepolymer so obtained is designated as "A-2". Various physical propertiesof "A-2" were measured as in Example 1.

EXAMPLE 3

In the reaction vessel, 257.5 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

A mixed solution of 0.585 g of methylene bisacrylamide and 7.865 g ofsodium methallylsulfonate in 532.6 g of a 50% aqueous solution ofacrylamide and 60 g of an aqueous solution in which 0.38 g of4,4'-azobis-4-cyanovaleric acid of 84% purity and NaOH in a molar amounttwice as much as the molar amount of the 4,4'-azobis-4-cyanovaleric acidhad been dissolved were prepared, respectively. Both the solutions wereadded dropwise to the reaction vessel at constant rates over 150minutes. In the course of the dropwise addition, the internaltemperature was maintained at 80° C.

After the completion of the dropwise addition, polymerization wascontinued for 3 hours at 80° C. Water was then added to the reactionvessel and the reaction vessel was cooled to terminate the reaction,thereby obtaining an aqueous solution of an acrylamide polymer having aBrookfield viscosity at 25° C. of 630 cps when the solution had anonvolatile content of 15% and having a Brookfield viscosity at 25° C.of 170 cps when the solution had a nonvolatile content of 10%. Thepolymer so obtained is designated as "A-3". Various physical propertiesof "A-3" were measured as in Example 1.

EXAMPLE 4

In the reaction vessel, 177.0 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

In 750.7 g of a 50% aqueous solution of acrylamide, 0.837 g of methylenebisacrylamide, 23.8 g of sodium methallylsulfonate and 0.544 g of4,4'-azobis-4-cyanovaleric acid of 84% purity were mixed and dissolvedto prepare a mixed monomer-initiator solution.

The mixed monomer-initiator solution so obtained was added dropwise tothe reaction vessel at a constant rate over 150 minutes. In the courseof the dropwise addition, the internal temperature of the reactionvessel was maintained at 80° C. The temperature of the mixedmonomer-initiator solution was, on the other hand, maintained at 20° C.or lower so that the polymerization did not occur before the dropwiseaddition.

After the completion of the dropwise addition, polymerization wascontinued for 3 hours at 80° C. Water was then added to the reactionvessel and the reaction vessel was cooled to terminate the reaction,thereby obtaining an aqueous solution of an acrylamide polymer having aBrookfield viscosity at 25° C. of 550 cps when the solution had anonvolatile content of 15% and having a Brookfield viscosity at 25° C.of 150 cps when the solution had a nonvolatile content of 10%. Thepolymer so obtained is designated as "A-4". Various physical propertiesof "A-4" were measured as in Example 1.

EXAMPLE 5

In the reaction vessel, 395.8 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

In 339.4 g of a 40% aqueous solution of acrylamide, 4.62 g of methylenebisacrylamide, 9.84 g of sodium methallylsulfonate and 0.9 g of4,4'-azobis-4-cyanovaleric acid of 84% purity were mixed and dissolvedto prepare a mixed monomer-initiator solution.

The mixed monomer-initiator solution so obtained was added dropwise tothe reaction vessel at a constant rate over 150 minutes. In the courseof the dropwise addition, the internal temperature of the reactionvessel was maintained at 80° C. The temperature of the mixedmonomer-initiator solution was, on the other hand, maintained at 20° C.or lower so that the polymerization did not occur before the dropwiseaddition.

After the completion of the dropwise addition, polymerization wascontinued for 3 hours at 80° C. Water was then added to the reactionvessel and the reaction vessel was cooled to terminate the reaction,thereby obtaining an aqueous solution of an acrylamide polymer having aBrookfield viscosity at 25° C. of 106 cps when the solution had anonvolatile content of 15% and having a Brookfield viscosity at 25° C.of 38 cps when the solution had a nonvolatile content of 10%. Thepolymer so obtained is designated as "A-5". Various physical propertiesof "A-5" were measured as in Example 1.

COMPARATIVE EXAMPLE 1

In the reaction vessel, 500 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

In 355.4 g of a 40% aqueous solution of acrylamide, 0.9 g of4,4'-azobis-4-cyanovaleric acid of 84% purity was dissolved. To theresultant solution, 60 g of water were added, whereby a mixedmonomer-initiator solution was prepared.

The mixed monomer-initiator solution so obtained was added dropwise tothe reaction vessel at a constant rate over 150 minutes. In the courseof the dropwise addition, the internal temperature of the reactionvessel was maintained at 80° C. The temperature of the mixedmonomer-initiator solution was, on the other hand, maintained at 20° C.or lower so that the polymerization did not occur before the dropwiseaddition.

After the completion of the dropwise addition, polymerization wascontinued for 3 hours at 80° C. Water was then added to the reactionvessel and the reaction vessel was cooled to terminate the reaction,thereby obtaining an aqueous solution of an acrylamide polymer having aBrookfield viscosity at 25° C. of 30,000 cps when the solution had anonvolatile content of 15% and having a Brookfield viscosity at 25° C.of 2,820 cps when the solution had a nonvolatile content of 10%. Thepolymer so obtained is designated as "C-1". Various physical propertiesof "C-1" were measured as in Example 1.

COMPARATIVE EXAMPLE 2

In the reaction vessel, 500 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

A solution in which 355.0 g of a 40% aqueous solution of acrylamide and0.308 g of methylene bisacrylamide were mixed and dissolved, and 60 g ofan aqueous solution of 0.18 g of 4,4'-azobis-4-cyanovaleric acid of 84%purity were prepared, respectively. Both the solutions were addeddropwise at constant rates to the reaction vessel as in Example 1, butthe reaction mixture lost its flowability in the course of the dropwiseaddition and at last, it gelated. The gelated polymer is designated as"C-2". It was impossible to measure the various properties of "C-2"because further dilution permitted neither dispersion nor dissolution of"C-2".

COMPARATIVE EXAMPLE 3

In the reaction vessel, 500 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

In 355.0 g of a 40% aqueous solution of acrylamide, 0.308 g of methylenebisacrylamide and 0.9 g of 4,4'-azobis-4-cyanovaleric acid of 84% puritywere dissolved. To the resultant solution, 60 g of water were added,whereby a mixed monomer-initiator solution was prepared.

The mixed monomer-initiator solution so obtained was added dropwise tothe reaction vessel at a constant rate over 150 minutes. In the courseof the dropwise addition, the internal temperature of the reactionvessel was maintained at 80° C. The temperature of the mixedmonomer-initiator solution was, on the other hand, maintained at 20° C.or lower so that the polymerization did not occur before the dropwiseaddition.

After the completion of the dropwise addition, polymerization wascontinued for 3 hours at 80° C. Water was then added to the reactionvessel and the reaction vessel was cooled to terminate the reaction,thereby obtaining an aqueous solution of an acrylamide polymer having aBrookfield viscosity at 25° C. of 15,000 cps when the solution had anonvolatile content of 15% and having a Brookfield viscosity at 25° C.of 1,400 cps when the solution had a nonvolatile content of 10%. Thepolymer so obtained is designated as "C-3". Various physical propertiesof "C-3" were measured as in Example 1.

COMPARATIVE EXAMPLE 4

In the reaction vessel, 395.8 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

In 348.3 g of a 40% aqueous solution of acrylamide, 4.62 g of methylenebisacrylamide and 0.9 g of 4,4'-azobis-4-cyanovaleric acid of 84% puritywere dissolved. The resulting solution was added dropwise it a constantrate to the reaction vessel as in Example 4, but the reaction mixturelost its flowability in the course of the dropwise addition and at last,it gelated. The gelated polymer is designated as "C-4". It wasimpossible to measure the various properties of "C-4", because furtherdilution permitted neither dispersion nor dissolution of "C-4".

Compositions and various properties of the polymers "A-1" to "A-5" and"C-1" to "C-4", which have been obtained in Examples 1-5 and ComparativeExamples 1-4, respectively are presented in Tables 1 and 2.

                  TABLE 1                                                         ______________________________________                                                                Polymerization                                        Composition (mole %)    initiator.sup.1)                                                                         Polymer                                    AM           SMS    MBA     (g/mole) No.                                      ______________________________________                                        Example 1                                                                             99.7     0.2    0.1   0.09     A-1                                    Example 2                                                                             99.35    0.25   0.4   0.45     A-2                                    Example 3                                                                             98.6     1.3    0.1   0.10     A-3                                    Example 4                                                                             97.13    2.77   0.1   0.10     A-4                                    Example 5                                                                             95.5     3.0    1.5   0.45     A-5                                    Comp. Ex. 1                                                                           100                   0.45     C-1                                    Comp. Ex. 2                                                                           99.9            0.1   0.09     C-2                                    Comp. Ex. 3                                                                           99.9            0.1   0.45     C-3                                    Comp. Ex. 4                                                                           98.5            0.5   0.45     C-4                                    ______________________________________                                         AM: Acrylamide                                                                SMS: Sodium methallylsulfonate                                                MBA: Methylene bisacrylamide                                                  1): 4,4Azobis-4-cyanovaleric acid                                        

                                      TABLE 2                                     __________________________________________________________________________                         Weight-           Percentage                                             Weight-                                                                            average   Number- of molecular                               Viscosity                                                                           Viscosity                                                                           average                                                                            root mean average weights of                                                                          Hydrated                             15%   10%   molecular                                                                          square    molecular                                                                             1,000,000                                                                           dia-                                 Product                                                                             Product                                                                             weight(a)                                                                          radius(b)                                                                          (b)  weight(c)                                                                          (a)                                                                              and higher                                                                          meter                            Polymer                                                                           (cps/25° C.)                                                                 (cps/25° C.)                                                                 (× 10,000)                                                                   (nm) (a)  (× 10,000)                                                                   (c)                                                                              (%)   (nm)                             __________________________________________________________________________    A-1 18000 1780  305  73.8 0.0000242                                                                          125  2.44                                                                             73.5  120                              A-2  2170  330  339  80.2 0.0000237                                                                          88.0 3.85                                                                             74.0  170                              A-3  630   170  312  72.2 0.0000231                                                                          113  2.76                                                                             71.8  240                              A-4  550   150  315  77.0 0.0000244                                                                          105  3.00                                                                             66.7  170                              A-5  106   38   206  61.5 0.0000299                                                                          51.3 4.02                                                                             41.4   93                              C-1 30000 2820  69.4 35.2 0.0000507                                                                          32.2 2.16                                                                             18.9   37                              C-2 Gelated                                                                   C-3 15000 1400  64.9 30.1 0.0000464                                                                          27.8 2.33                                                                             18.4   28                              C-4 Gelated                                                                   __________________________________________________________________________     Hydrated diameters of "C1" and "C3" are practically unmeasurable, because     each of the polymers had a scattering strength (counter number) of 50,000     or smaller when measured by the N4type analyzer of Coulter Inc. In the        above table, values at scattering strength of 50000 or smaller are shown      for reference although they are inherently unmeasurable.                 

EXAMPLE 6

In the reaction vessel, 600 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

In 332.8 g of a 40% aqueous solution of acrylamide, 0.308 g of methylenebisacrylamide, 0.79 g of sodium methallylsulfonate, 12.6 g ofN,N-dimethylaminoethyl methacrylate and 5.2 g of itaconic acid weremixed and dissolved. The resulting solution was adjusted to pH 4.2 with35% HCl.

Also prepared were 60 g of an aqueous solution in which 0.16 g of4,4'-azobis-4-cyanovaleric acid of 84% purity was dissolved.

Both the solutions were added dropwise to the reaction vessel atconstant rates over 150 minutes, respectively. In the course of thedropwise addition, the internal temperature of the reaction vessel wasmaintained at 80° C. After the completion of the dropwise addition,polymerization was continued for 3 hours at 80° C. The reaction vesselwas then cooled to terminate the reaction. Water was added to theresulting polymer for the adjustment of a nonvolatile content, therebyobtaining the aqueous solution of an acrylamide polymer having aBrookfield viscosity at 25° C. of 5,000 cps when the solution had anonvolatile content of 15% and a Brookfield viscosity at 25° C. of 960cps when the solution had a nonvolatile content of 10%. The polymer soobtained is designated as "A-6". Various physical properties of "A-6"were measured as in Example 1.

EXAMPLE 7

In the reaction vessel, 660 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

In 331.8 g of a 40% aqueous solution of acrylamide, 1.23 g of methylenebisacrylamide, 0.79 g of sodium methallylsulfonate, 12.6 g ofN,N-dimethylaminoethyl methacrylate, 5.2 g of itaconic acid and 0.9 g of4,4'-azobis-4-cyanovaleric acid of 84% purity were mixed and dissolved.The resulting solution was subjected to pH adjustment with 35% HCl,whereby a mixed monomer-initiator solution of pH 4.2 was obtained.

The mixed monomer-initiator solution so obtained was added dropwise tothe reaction vessel at a constant rate over 150 minutes, respectively.In the course of the dropwise addition, the internal temperature of thereaction vessel was maintained at 80° C. The temperature of the mixedmonomer-initiator solution was maintained at 20° C. or lower so that thepolymerization did not occur before the dropwise addition.

After the completion of the dropwise addition, polymerization wascontinued for 3 hours at 80° C. The reaction vessel was then cooled toterminate the reaction. Water was added to the resulting polymer for theadjustment of a nonvolatile content, thereby obtaining an aqueoussolution of an acrylamide polymer having a Brookfield viscosity at 25°C. of 3,000 cps when the solution had a nonvolatile content of 15% and aBrookfield viscosity at 25° C. of 620 cps when the solution had anonvolatile content of 10%. The polymer so obtained is designated as"A-7". Various physical properties of "A-7" were measured as in Example1.

EXAMPLE 8

In the reaction vessel, 580 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

In 325.7 g of a 40% aqueous solution of acrylamide, 0.308 g of methylenebisacrylamide, 0.79 g of sodium methallylsulfonate, 12.5 g ofN,N-dimethylaminopropyl acrylamide and 7.2 g of 80% acrylic acid weremixed and dissolved. The resulting solution was adjusted to pH 4.2 with35% HCl.

Also prepared were 60 g of an aqueous solution in which 0.16 g of4,4'-azobis-4-cyanovaleric acid of 84% purity was dissolved.

Both the solutions were added dropwise to the reaction vessel atconstant rates over 150 minutes, respectively. In the course of thedropwise addition, the internal temperature of the reaction vessel wasmaintained at 80° C. After the completion of the dropwise addition,polymerization was continued for 3 hours at 80° C. The reaction vesselwas then cooled to terminate the reaction. Water was added to theresulting polymer for the adjustment of a nonvolatile content, therebyobtaining an aqueous solution of an acrylamide polymer having aBrookfield viscosity at 25° C. of 11,000 cps when the solution had anonvolatile content of 15% and a Brookfield viscosity at 25° C. of 1980cps when the solution had a nonvolatile content of 10%. The polymer soobtained is designated as "A-8". Various physical properties of "A-8"were measured as in Example 1.

EXAMPLE 9

In the reaction vessel, 387 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

In 414.6 g of a 40% aqueous solution of acrylamide, 0.385 g of methylenebisacrylamide, 2.25 g of sodium methallylsulfonate, 15.7 g ofN,N-dimethylaminoethyl methacrylate and 6.5 g of itaconic acid weremixed and dissolved. The resulting solution was adjusted to pH 4.2 with35% HCl.

Also prepared were 60 g of an aqueous solution in which 0.25 g of4,4'-azobis-4-cyanovaleric acid of 84% purity and NaOH in a mole twiceas much as the mole of the 4,4'-azobis-4-cyanovaleric acid weredissolved.

Both the solutions were added dropwise to the reaction vessel atconstant rates over 150 minutes, respectively. In the course of thedropwise addition, the internal temperature of the reaction vessel wasmaintained at 80° C. After the completion of the dropwise addition,polymerization was continued for 3 hours at 80° C. The reaction vesselwas then cooled to terminate the reaction. Water was added to theresulting polymer for the adjustment of a nonvolatile content, therebyobtaining an aqueous solution of an acrylamide polymer having aBrookfield viscosity at 25° C. of 1,500 cps when the solution had anonvolatile content of 15% and a Brookfield viscosity at 25° C. of 340cps when the solution had a nonvolatile content of 10%. The polymer soobtained is designated as "A-9". Various physical properties of "A-9"were measured as in Example 1.

COMPARATIVE EXAMPLE 5

In the reaction vessel, 660 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

In 333.7 g of a 40% aqueous solution of acrylamide, 0.308 g of methylenebisacrylamide, 12.6 g of N,N-dimethylaminoethyl methacrylate, 5.2 g ofitaconic acid and 0.9 g of 4,4'-azobis-4-cyanovaleric acid of 84% puritywere mixed and dissolved. The resulting solution was adjusted to pH 4.2with 35% HCl.

The solution so adjusted was added dropwise to the reaction vessel at aconstant rate over 150 minutes, respectively. In the course of thedropwise addition, the internal temperature of the reaction vessel wasmaintained at 80° C.

After the completion of the dropwise addition, polymerization wascontinued for 3 hours at 80° C. The reaction vessel was then cooled toterminate the reaction. Water was added to the resulting polymer for theadjustment of a nonvolatile content, thereby obtaining an aqueoussolution of an acrylamide polymer having a Brookfield viscosity at 25°C. of 9,200 cps when the solution had a nonvolatile content of 15% and aBrookfield viscosity at 25° C. of 630 cps when the solution had anonvolatile content of 10%. The polymer so obtained is designated as"C-5". Various physical properties of "C-5" were measured as in Example1.

COMPARATIVE EXAMPLE 6

In the reaction vessel, 600 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

In 333.7 g of a 40% aqueous solution of acrylamide, 0.308 g of methylenebisacrylamide, 12.6 g of N,N-dimethylaminoethyl methacrylate and 5.2 gof itaconic acid were mixed and dissolved. The resulting solution wasadjusted to pH 4.2 with 35% HCl.

Also prepared were 60 g of an aqueous solution in which 0.16 g of4,4'-azobis-4-cyanovaleric acid of 84% purity was dissolved.

Both the solutions were added dropwise at constant rates to the reactionvessel as in Example 6, but the reaction mixture lost its flowability inthe course of the dropwise addition and at last, it gelated. The gelatedpolymer is designated as "C-6". It was impossible to measure the variousproperties of "C-6" because further dilution permitted neitherdispersion nor dissolution of "C-6".

COMPARATIVE EXAMPLE 7

In the reaction vessel, 333.7 g of a 40% aqueous solution of acrylamide,0.308 g of methylene bisacrylamide, 12.6 g of N,N-dimethylaminoethylmethacrylate, 5.2 g of itaconic acid and 660 g of water were charged.After they were stirred into a solution, the solution was adjusted to pH4.2 with 35% HCl.

The solution so adjusted in the reaction vessel was subjected todeoxidation with N₂ gas. After the resulting solution was heated to 45°C., the reaction vessel was lagged.

Under stirring, 1.32 g of ammonium persulfate and 0.6 g of sodiumbisulfite were added to the resulting solution to initiatepolymerization. Sixty minutes after, the temperature of the solutionreached 80° C. The solution was then left alone at 80° C. for one hour,followed by cooling to terminate the reaction. Water was then added tothe resulting polymer for the adjustment of a nonvolatile content,thereby obtaining an aqueous solution of an acrylamide polymer having aBrookfield viscosity at 25° C. of 8,700 cps when the solution had anonvolatile content of 15% and a Brookfield viscosity at 25° C. of 1,700cps when the solution had a nonvolatile content of 10%. The polymer soobtained is designated as "C-7". Various physical properties of "C-7"were measured as in Example 1.

COMPARATIVE EXAMPLE 8

In the reaction vessel, 660 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

In 332.7 g of a 40% aqueous solution of acrylamide, 1.23 g of methylenebisacrylamide, 12.6 g of N,N-dimethylaminoethyl methacrylate, 5.2 g ofitaconic acid and 0.9 g of 4,4'-azobis-4-cyanovaleric acid of 84% puritywere mixed and dissolved. The resulting solution was adjusted to pH 4.2with 35% HCl.

The solution was added dropwise at a constant rate to the reactionvessel as in Example 7, but the reaction mixture lost its flowability inthe course of the dropwise addition and at last, it gelated. The gelatedpolymer is designated as "C-8". It was impossible to measure the variousproperties of "C-8" because further dilution permitted neitherdispersion nor dissolution of "C-8".

COMPARATIVE EXAMPLE 9

In the reaction vessel, 660 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

In 332.7 g of a 40% aqueous solution of acrylamide, 1.23 g of methylenebisacrylamide, 12.6 g of N,N-dimethylaminoethyl methacrylate, 5.2 g ofitaconic acid, 9.85 g of allyl alcohol as a molecular weight modifierand 0.9 g of 4,4'-azobis-4-cyanovaleric acid of 84% purity were mixedand dissolved. The resulting solution was adjusted to pH 4.2 with 35%HCl.

The solution so adjusted was added dropwise to the reaction vessel at aconstant rate over 150 minutes. In the course of the dropwise addition,the internal temperature of the reaction vessel was maintained at 80° C.

After the completion of the dropwise addition, polymerization wascontinued for 3 hours at 80° C. The reaction vessel was then cooled toterminate the reaction. Water was added to the resulting polymer for theadjustment of a nonvolatile content, thereby obtaining an aqueoussolution of an acrylamide polymer having a Brookfield viscosity at 25°C. of 10,000 cps when the solution had a nonvolatile content of 15% anda Brookfield viscosity at 25° C. of 1,860 cps, when the solution had anonvolatile content of 10%. the polymer so obtained is designated as"C-9". Various physical properties of "C-9" were measured as in Example1.

COMPARATIVE EXAMPLE 10

In the reaction vessel, 560 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

On the other hand, 325.7 g of a 40% aqueous solution of acrylamide,0.308 g of methylene bisacrylamide, 2.6 g of a 25% aqueous solution ofsodium vinylsulfonate, 12.5 g of N,N-dimethylaminopropyl acrylamide, 7.2g of 80% acrylic acid and 23 g of isopropyl alcohol were stirred into asolution. The resulting solution was adjusted to pH 4.2 with 35% HCl.

Also prepared were 60 g of an aqueous solution in which 0.16 g of4,4'-azobis-4-cyanovaleric acid of 84% purity was dissolved.

Both the solutions were added dropwise to the reaction vessel atconstant rates over 150 minutes. In the course of the dropwise addition,the internal temperature of the reaction vessel was maintained at 80° C.After the completion of the dropwise addition, polymerization wascontinued for 3 hours at 80° C. The reaction vessel was then cooled toterminate the reaction. Water was added to the resulting polymer for theadjustment of a nonvolatile content, thereby obtaining an aqueoussolution of an acrylamide polymer having a Brookfield viscosity at 25°C. of 6,800 cps when the solution had a nonvolatile content of 15% and aBrookfield viscosity at 25° C. of 1,170 cps when the solution had anonvolatile content of 10%. The polymer so obtained is designated as"C-10". Various physical properties of "C-10" were measured as inExample 1.

Compositions and various properties of the polymers "A-6" to "A-9" and"C-5" to "C-10", which have been obtained in Examples 6-9 andComparative Examples 5-10, respectively are presented in Tables 3 and 4.

                                      TABLE 3                                     __________________________________________________________________________                                 Polymeri-                                                                     zation                                                  Composition           initiator.sup.1)                                                                     Polymer                                          (mole %)              (g/mole)                                                                             No.                                       __________________________________________________________________________    Example 6                                                                            AM 93.7                                                                            DM 4 IA 2                                                                             SMS 0.25                                                                           MBA 0.1                                                                            0.08  A-6                                       Example 7                                                                            AM 93.35                                                                           DM 4 IA 2                                                                             SMS 0.25                                                                           MBA 0.4                                                                            0.45  A-7                                       Example 8                                                                            AM 91.65                                                                           DMAPA 4                                                                            AA 4                                                                             SMS 0.25                                                                           MBA 0.1                                                                            0.08  A-8                                       Example 9                                                                            AM 93.33                                                                           DM 4 IA 2                                                                             SMS 0.57                                                                           MBA 0.1                                                                            0.10  A-9                                       Comp. Ex. 5                                                                          AM 93.9                                                                            DM 4 IA 2    MBA 0.1                                                                            0.45  C-5                                       Comp. Ex. 6                                                                          AM 93.9                                                                            DM 4 IA 2    MBA 0.1                                                                            0.08  C-6                                       Comp. Ex. 7                                                                          AM 93.9                                                                            DM 4 IA 2    MBA 0.1                                                                            Redox type                                                                          C-7                                       Comp. Ex. 8                                                                          AM 93.6                                                                            DM 4 IA 2    MBA 0.4                                                                            0.45  C-8                                       Comp. Ex. 9                                                                          AM 93.6                                                                            DM 4 IA 2    MBA 0.4                                                                            0.45.sup.2)                                                                         C-9                                       Comp. Ex. 10                                                                         AM 91.65                                                                           DMAPA 4                                                                            AA 4                                                                             VS 0.25                                                                            MBA 0.1                                                                            0.08.sup.3)                                                                          C-10                                     __________________________________________________________________________     DM: N,NDimethylaminoethyl methacrylate, IA: Itaconic acid, DMAPA:             N,NDimethylaminopropyl acrylamide, AA: Acrylic acid, VS: Sodium               vinylsulfonate                                                                .sup.1) 4,4Azobis-4-cyanovaleric acid                                         .sup.2) Allyl alcohol was used.                                               .sup.3) Isopropyl alcohol was used.                                      

                                      TABLE 4                                     __________________________________________________________________________                         Weight-           Percentage                                             Weight-                                                                            average   Number- of molecular                               Viscosity                                                                           Viscosity                                                                           average                                                                            root mean average weights of                                                                          Hydrated                             15%   10%   molecular                                                                          square    molecular                                                                             1,000,000                                                                           dia-                                 Product                                                                             Product                                                                             weight(a)                                                                          radius(b)                                                                          (b)  weight(c)                                                                          (a)                                                                              and higher                                                                          meter                            Polymer                                                                           (cps/25° C.)                                                                 (cps/25° C.)                                                                 (× 10,000)                                                                   (nm) (a)  (× 10,000)                                                                   (c)                                                                              (%)   (nm)                             __________________________________________________________________________    A-6 5000   960  359  94.6 0.0000264                                                                          131  2.74                                                                             74.5  140                              A-7 3000   620  407  93.1 0.0000229                                                                          104  3.91                                                                             65.2  200                              A-8 11000 1980  340  90.6 0.0000266                                                                          123  2.76                                                                             73.0  150                              A-9 1500   340  292  75.0 0.0000257                                                                          101  2.89                                                                             65.7  140                              C-5 9200  1630  84.5 38.8 0.0000459                                                                          36.9 2.29                                                                             27.8   86                              C-6 Gelated                                                                   C-7 8700  1700  91.0 49.9 0.0000548                                                                          39.0 2.33                                                                             26.9  156                              C-8 Gelated                                                                   C-9 10000 1860  121  57.1 0.0000472                                                                          41.0 2.95                                                                             33.1   65                              C-10                                                                              6800  1170   90  43.0 0.0000478                                                                          35.0 2.57                                                                             26.4   78                              __________________________________________________________________________     Hydrated diameters of "C5" to "C10" are practically unmeasurable, because     each of the polymers had a scattering strength (counter number) of 50,000     or smaller as measured by the N4type analyzer of Coulter Inc. In the abov     table, values at the scattering strength of 50000 or smaller are shown fo     reference although they are inherently unmeasurable.                     

Application examples will be described next. In each case, the polymerobtained in each of the Examples and Comparative Examples was utilizedas an internal paper strength agent.

APPLICATION EXAMPLE 1

To a 1% slurry of L-BKP which had a beating degree of 400 ml in terms ofCSF (Canadian Standard Freeness), aluminum sulfate was added in anamount of 0.5% based on the amount of the pulp, followed by stirring for3 minutes. The pulp slurry at that time had pH of 6.0. Under stirring, a1% aqueous solution of Polymer "A-6" was added in an amount of 0.5%based on the amount of the pulp on a nonvolatile basis. Stirring wascontinued further for 3 minutes. Using the pulp slurry so obtained,freeness was measured (JIS-P8112) and paper was formed by a TAPPI squaresheet machine. The wet sheet so obtained was dried at 110° C. for 3minutes in a drum drier, whereby a hand-made paper of 100 g/m² in basisweight was obtained. The paper so dried was subjected to moistureconditioning for at least 24 hours in an air-conditioned chamber of 20°C. and RH 65%, followed by measurement of burst index (JIS-P8112) andinternal bond strength (JAPAN TAPPI54). Polymers "A-7"-"A-9", "C-5","C-7", "C-9" and "C-10" were processed likewise. The results are shownin Table 5.

                  TABLE 5                                                         ______________________________________                                                      Freeness  Burst  Internal bond                                         Polymer                                                                              (ml)      index  strength (kg · cm)                    ______________________________________                                                 Not added                                                                              420       3.30 3.15                                         Example  A-6      460       4.44 5.90                                                  A-7      480       4.50 6.00                                                  A-8      455       4.40 5.91                                                  A-9      450       4.35 5.88                                         Comparative                                                                            C-5      430       3.90 4.20                                         Example  C-7      445       3.88 4.50                                                  C-9      440       3.87 4.44                                                   C-10    440       3.94 4.30                                         ______________________________________                                    

It is evident that each acrylamide polymer according to the presentinvention has physical properties and a structure as shown in Tables 2and 4. In some of the comparative examples, an acrylamide polymer wasprepared by the prior art technique. Any polymerization processaccording to the prior art technique cannot provide a polymer havingsuch a novel structure and excellent physical properties.

As shown in application examples (Table 5), the acrylamide polymers ofthe present invention hence have an excellent paper strength producingeffect, which is not available from the conventional art.

EXAMPLE 10

In the reaction vessel, 306 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

On the other hand, a solution of 0.31 g of methylene bisacrylamide and1.58 g of sodium methallylsulfonate in 283 g of a 50% aqueous solutionof acrylamide and 60 g of an aqueous solution in which 0.24 g of4,4'-azobis-4-cyanovaleric acid of 84% purity had been dissolved wereprepared. Both the solutions were added dropwise to the reaction vesselat constant rates over 150 minutes. In the course of the dropwiseaddition, the internal temperature of the reaction vessel was kept at80° C. After the completion of the dropwise addition, polymerization wascontinued for 3 hours at 80° C. The reaction vessel was then cooled toterminate the reaction, thereby obtaining an aqueous solution of anacrylamide polymer having a Brookfield viscosity at 25° C. of 12,000cps. The polymer so obtained is designated as "A-10". In an aluminum cupwhose weight was already known, 1 g of "A-10" was weighed precisely. Thepolymer was diluted with about 1 g of purified water, followed by dryingfor three hours by a hot-air drier of 105° C. to determine its oven-drypolymer concentration. It was found to be 23.8%. Further, as a result ofthe measurement of the weight-average molecular weight of "A-10" inaccordance with the method described above, it was found to be1,760,000.

EXAMPLE 11

In the reaction vessel, 288 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

In 283 g of a 50% aqueous solution of acrylamide, 0.31 g of methylenebisacrylamide, 1.58 g of sodium methallylsulfonate and 0.24 g of4,4'-azobis-4-cyanovaleric acid of 84% purity were mixed and dissolved,whereby a mixed monomer-initiator solution was prepared.

The mixed monomer-initiator solution so obtained was added dropwise tothe reaction vessel at constant rates over 150 minutes. In the course ofthe dropwise addition, the internal temperature of the reaction vesselwas kept at 80° C. The temperature of the mixed monomer-initiatorsolution was maintained at 15-25° C. as a precaution for avoidingpolymerization or precipitation of the monomers before the dropwiseaddition. Whether polymerization of the mixed monomer-initiator solutionhad occurred before the dropwise addition or not was confirmed byleaving the mixed solution alone for 2 hours and then adding it intomethanol.

After the completion of the dropwise addition, polymerization wascontinued for 3 hours at 80° C. The reaction vessel was then cooled toterminate the reaction, thereby obtaining an aqueous solution of anacrylamide polymer having a Brookfield viscosity at 25° C. of 3,340 cps.The polymer so obtained is designated as "A-11". Various physicalproperties of "A-11" were measured as in Example 1.

EXAMPLE 12

In the reaction vessel, 293 g of purified water were charged. Whilenitrogen gas was blown into the reaction vessel, the internaltemperature was adjusted to 80° C.

In 2823 g of a 50% aqueous solution of acrylamide, 0.31 g of methylenebisacrylamide, 3.18 g of sodium methallylsulfonate and 0.25 g of2,2'-azobis-2-methyl-N-(2-hydroxyethyl)-propionamide were mixed anddissolved to prepare a mixed monomer-initiator solution. The solution soobtained was added dropwise to the reaction vessel at a constant rateover 150 minutes. In the course of the dropwise addition, the internaltemperature of the reaction vessel was kept at 80° C. The temperature ofthe mixed monomer-initiator solution was maintained at 15-25° C.

After the completion of the dropwise addition, polymerization wascontinued for 3 hours at 80° C. The reaction vessel was then cooled toterminate the reaction, thereby obtaining an aqueous solution of anacrylamide polymer having a Brookfield viscosity at 25° C. of 3,700 cps.The polymer so obtained is designated as "A-12". Various physicalproperties of "A-12" were measured as in Example 1.

EXAMPLE 13

In the reaction vessel, 296 g of purified water were charged. Whilenitrogen gas was blown into the reaction vessel, the internaltemperature was adjusted to 80° C.

In 421 g of a 50% aqueous solution of acrylamide, 0.46 g of methylenebisacrylamide, 6.22 g of sodium methallylsulfonate and 0.36 g of4,4'-azobis-4-cyanovaleric acid of 84% purity were mixed and dissolved,whereby a mixed monomer-initiator solution was prepared. The resultingsolution was then added dropwise to the reaction vessel-at a constantrate over 150 minutes. In the course of the dropwise addition, theinternal temperature of the reaction vessel was kept at 80° C. Thetemperature of the mixed monomer-initiator solution was maintained at15-25° C.

After the completion of the dropwise addition, polymerization wascontinued for 3 hours at 80° C. The reaction vessel was then cooled toterminate the reaction, thereby obtaining an aqueous solution of anacrylamide polymer having a Brookfield viscosity at 25° C. of 20,000cps. The polymer so obtained is designated as "A-13". Various physicalproperties of "A-13" were measured as in Example 1.

EXAMPLE 14

In the reaction vessel, 325 g of purified water were charged. Whilenitrogen gas was blown into the reaction vessel, the internaltemperature was adjusted to 80° C.

In 550 g of a 50% aqueous solution of acrylamide, 14.4 g of methylenebisacrylamide, 111 g of sodium methallylsulfonate and 0.93 g of4,4'-azobis-4-cyanovaleric acid of 84% purity were mixed and dissolved,whereby a mixed monomer-initiator solution was prepared.

The mixed monomer-initiator solution so obtained was added dropwise tothe reaction vessel at a constant rate over 150 minutes. In the courseof the dropwise addition, the internal temperature of the reactionvessel was kept at 80° C. The temperature of the mixed monomer-initiatorsolution was maintained at 15-25° C.

After the completion of the dropwise addition, polymerization wascontinued for 3 hours at 80° C. The reaction vessel was then cooled toterminate the reaction, thereby obtaining an aqueous solution of anacrylamide polymer having a Brookfield viscosity at 25° C. of 1,070 cps.The polymer so obtained is designated as "A-14". Various physicalproperties of "A-14" were measured as in Example 1.

EXAMPLE 15

In the reaction vessel, 203 g of purified water were charged. Whilenitrogen gas was blown into the reaction vessel, the internaltemperature was adjusted to 80° C.

In 747 g of a 60% aqueous solution of acrylamide, 1.02 g of methylenebisacrylamide, 52.8 g of sodium methallylsulfonate and 0.66 g of4,4'-azobis-4-cyanovaleric acid of 84% purity were mixed and dissolved,whereby a mixed monomer-initiator solution was prepared.

The mixed monomer-initiator solution so obtained was added dropwise tothe reaction vessel at a constant rate over 150 minutes. In the courseof the dropwise addition, the internal temperature of the reactionvessel was kept at 80° C. and the temperature of the mixedmonomer-initiator solution was maintained at 15-25° C.

After the completion of the dropwise addition, polymerization wascontinued for 3 hours at 80° C. The reaction vessel was then cooled toterminate the reaction, thereby obtaining an aqueous solution of anacrylamide polymer having a Brookfield viscosity at 25° C. of 47,600cps. The polymer so obtained is designated as "A-15". Various physicalproperties of "A-15" were measured as in Example 1.

COMPARATIVE EXAMPLE 11

In the reaction vessel, 363 g of purified water were charged. Whilenitrogen gas was blown into the reaction vessel, the internaltemperature was adjusted to 80° C.

In 284 g of a 50% aqueous solution of acrylamide, 0.31 g of methylenebisacrylamide and 0.24 g of 4,4'-azobis-4-cyanovaleric acid of 84%purity were mixed and dissolved. The resulting aqueous solution wasadded dropwise to the reaction vessel at a constant rate over 150minutes. In the course of the dropwise addition, the internaltemperature of the reaction vessel was maintained at 80° C. and thetemperature of the mixed solution was maintained at 15-25° C. In thecourse of the dropwise addition, however, the reaction mixture becamehighly viscous, lost its flowability and finally gelated. The gelatedpolymer is designated as "C-11". It was impossible to measure thevarious properties of "C-11" because further dilution with waterpermitted neither dispersion nor dissolution of "C-11".

COMPARATIVE EXAMPLE 12

In the reaction vessel, 228 g of purified water were charged. Whilenitrogen gas was blown into the reaction vessel, the internaltemperature was adjusted to 80° C.

A solution in which 283 g of a 50% aqueous solution of acrylamide, 0.31g of methylene bisacrylamide and 30 g of isopropyl alcohol had beenmixed and 60 g of an aqueous solution of 0.24 g of4,4'-azobis-4-cyanovaleric acid of 84% purity were added dropwise to thereaction vessel at constant rates over 150 minutes, respectively. In thecourse of the dropwise addition, the internal temperature of thereaction vessel was kept at 80° C. After the completion of the dropwiseaddition, polymerization was continued for 3 hours at 80° C. Thereaction vessel was then cooled to terminate the reaction, therebyobtaining an aqueous solution of an acrylamide polymer having aBrookfield viscosity at 25° C. of 13,300 cps. The polymer so obtained isdesignated as "C-12". Various physical properties of "C-12" weremeasured as in Example 1.

COMPARATIVE EXAMPLE 13

In the reaction vessel, 217 g of purified water were charged. Whilenitrogen gas was blown into the reaction vessel, the internaltemperature was adjusted to 80° C.

In 766 g of a 50% aqueous solution of acrylamide, 17.0 g of methylenebisacrylamide and 1.96 g of 4,4'-azobis-4-cyanovaleric acid of 84%purity were mixed and dissolved, whereby a mixed monomer-initiatorsolution was obtained. The resulting solution was added dropwise to thereaction vessel at a constant rate over 150 minutes. In the course ofthe dropwise addition, the internal temperature of the reaction vesselwas maintained at 80° C. and the temperature of the mixed solution wasmaintained at 15-25° C. In the course of the dropwise addition, however,the reaction mixture became highly viscous, lost its flowability andfinally gelated. The gelated polymer is designated as "C-13". It wasimpossible to measure the various properties of "C-13" because furtherdilution with water permitted neither dispersion nor dissolution of"C-13".

COMPARATIVE EXAMPLE 14

In the reaction vessel, 167 g of purified water were charged. Whilenitrogen gas was blown into the reaction vessel, the internaltemperature was adjusted to 80° C.

In 8312 g of a 60% aqueous solution of acrylamide, 1.1 g of methylenebisacrylamide and 0.84 g of 4,4'-azobis-4-cyanovaleric acid of 84%purity were mixed and dissolved, whereby a mixed monomer-initiatorsolution was obtained. The resulting solution was added dropwise to thereaction vessel at a constant rate over 150 minutes. In the course ofthe dropwise addition, the internal temperature of the reaction vesselwas maintained at 80° C. and the temperature of the mixed solution wasmaintained at 15-25° C. In the course of the dropwise addition, however,the reaction mixture became highly viscous, lost its flowability andfinally gelated. The gelated polymer is designated as "C-14". It wasimpossible to measure the various properties of "C-14" because furtherdilution with water permitted neither dispersion nor dissolution of"C-14".

Compositions and various properties of the polymers "A-10" to "A-15" and"C-11" to "C-14", which have been obtained in Examples 10-15 andComparative Examples 11-14, respectively are presented in Table 6.

                                      TABLE 6                                     __________________________________________________________________________            Examples          Comparative Examples                                        10 11 12 13 14 15 11 12 13 14                                         __________________________________________________________________________    AM (mole %)                                                                           99.4                                                                             99.4                                                                             98.9                                                                             98.6                                                                             83 94.9                                                                             99.9                                                                             99.9                                                                             98.0                                                                             99.9                                       MBA (mole %)                                                                          0.1                                                                              0.1                                                                              0.1                                                                              0.1                                                                              2.0                                                                              0.1                                                                              0.1                                                                               0.1                                                                             2.0                                                                              0.1                                        SMS (mole %)                                                                          0.5                                                                              0.5                                                                              1.0                                                                              1.3                                                                              15.0                                                                             5.0                                                                              0  0  0  0                                          Initiator.sup.1)                                                                      A  A  B  A  A  A  A  A  A  A                                          Amount g/M.sup.2)                                                                     0.1                                                                              0.1                                                                               0.12                                                                            0.1                                                                              0.2                                                                              0.1                                                                              0.1                                                                               0.1                                                                             0.3                                                                              0.1                                        DPC.sup.3) (%)                                                                        23.8                                                                             27.4                                                                             27.4                                                                             31.9                                                                             43.1                                                                             53.9                                                                             -- 25.6                                                                             -- --                                         Viscosity (ps)                                                                        120                                                                              33.4                                                                             37.7                                                                             200                                                                              10.7                                                                             476                                                                              Gel                                                                              133                                                                              Gel                                                                              Gel                                        Mw.sup.4) (× 10,000)                                                            176                                                                              90.5                                                                             130                                                                              312                                                                              77.3                                                                              62                                                                              -- 36.6                                                                             -- --                                         __________________________________________________________________________     .sup.1) Initiator                                                             .sup.  A: 4,4Aazobis-4-cyanovaleric acid                                      .sup.  B: 2,2Azobis-2-methyl-N-(2-hydroxyethyl)-propionamide                  .sup.2) Amount of the initiator: gram number per mole of the monomer(s)       employed                                                                      .sup.3) DPC: Ovendry polymer concentration                                    .sup.4) Mw: weightaverage molecular weight                               

EXAMPLE 16

In the reaction vessel, 355 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

In 263 g of a 50% aqueous solution of acrylamide, 0.25 g of methylenebisacrylamide, 1.49 g of sodium methallylsulfonate, 15.7 g ofN,N-dimethylaminoethyl methacrylate and 5.2 g of itaconic acid weremixed and dissolved. The resulting solution was adjusted to pH 4.2 with35% HCl.

Also prepared were 60 g of an aqueous solution in which 0.24 g of4,4'-azobis-4-cyanovaleric acid of 84% purity was dissolved.

Both the solutions were added dropwise to the reaction vessel atconstant rates over 150 minutes, respectively. In the course of thedropwise addition, the internal temperature of the reaction vessel wasmaintained at 80° C. After the completion of the dropwise addition,polymerization was continued for 3 hours at 80° C. The reaction vesselwas then cooled to terminate the reaction, thereby obtaining the aqueoussolution of an acrylamide polymer having a Brookfield viscosity at 25°C. of 8,950 cps. The polymer so obtained is designated as "A-16".Various physical properties of "A-16" were measured as in Example 1.

EXAMPLE 17

In the reaction vessel, 286 g of purified water were charged. Whilenitrogen gas was blown into the flask, the internal temperature wasadjusted to 80° C.

In 394 g of a 50% aqueous solution of acrylamide, 0.46 g of methylenebisacrylamide, 6.30 g of sodium methallylsulfonate, 18.8 g ofN,N-dimethylaminopropyl acrylamide, 5.94 g of itaconic acid and 5.48 gof acrylic acid were mixed and dissolved. The resulting solution wasadjusted to pH 4.2 with 35% HCl.

Also prepared were 60 g of an aqueous solution in which 0.36 g of4,4'-azobis-4-cyanovaleric acid of 84% purity was dissolved.

Both the solutions were added dropwise to the reaction vessel atconstant rates over 150 minutes, respectively. In the course of thedropwise addition, the internal temperature of the reaction vessel wasmaintained at 80° C. After the completion of the dropwise addition,polymerization was continued for 3 hours at 80° C. The reaction vesselwas then cooled to terminate the reaction, thereby obtaining the aqueoussolution of an acrylamide polymer having a Brookfield viscosity at 25°C. of 12,000 cps. The polymer so obtained is designated as "A-17".Various physical properties of "A-17" were measured as in Example 1.

EXAMPLE 18

In the reaction vessel, 173 g of purified water were charged. Whilenitrogen gas was blown into the flask, the internal temperature wasadjusted to 80° C.

In 730 g of a 50% aqueous solution of acrylamide, 1.67 g of methylenebisacrylamide, 25.7 g of sodium methallylsulfonate and 9.75 g of acrylicacid were mixed and dissolved. The resulting solution was adjusted to pH4.2 with 40% sulfuric acid.

Also prepared were 60 g of an aqueous solution in which 1.08 g of2,2'-azobis-2-methylpropionamidine hydrochloride were dissolved.

Both the solutions were added dropwise to the reaction vessel atconstant rates over 150 minutes, respectively. In the course of thedropwise addition, the internal temperature of the reaction vessel wasmaintained at 80° C. After the completion of the dropwise addition,polymerization was continued for 3 hours at 80° C. The reaction vesselwas then cooled to terminate the reaction, thereby obtaining the aqueoussolution of an acrylamide polymer having a Brookfield viscosity at 25°C. of 25,500 cps. The polymer so obtained is designated as "A-18".Various physical properties of "A-18" were measured as in Example 1.

COMPARATIVE EXAMPLE 15

In the reaction vessel, 409 g of purified water, 263 g of a 50% aqueoussolution of acrylamide, 0.19 g of methylene bisacrylamide, 15.7 g ofN,N-dimethylaminoethyl methacrylate and 5.21 g of itaconic acid werecharged. After they were stirred into a solution, the solution wasadjusted to pH 4.2 with 35% HCl. The solution so adjusted was heated to35° C., followed by the addition of 1.0 g of2,2'-azobis-2-(2-imidazolin-2-yl)propane hydrochloride. Twenty minutesafter the addition, the initiation of the polymerization was confirmed.Twenty minutes after the initiation, the internal temperature of thereaction vessel reached 90° C. The polymerization was continued for 2hours at that temperature. The reaction vessel was then cooled toterminate the reaction, thereby obtaining the aqueous solution of anacrylamide polymer having a Brookfield viscosity at 25° C. of 46,100cps. The polymer so obtained is designated as "C15". Various physicalproperties of "C-15" were measured as in Example 1.

COMPARATIVE EXAMPLE 16

In the reaction vessel, 278 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

In 397 g of a 50% aqueous solution of acrylamide, 0.31 g of methylenebisacrylamide, 23.5 g of N,N-dimethylaminoethyl methacrylate and 7.8 gof itaconic acid were mixed and dissolved. The resulting solution wasadjusted to pH 4.2 with 35% HCl.

Also prepared were 60 g of an aqueous solution in which 1.07 g of4,4'-azobis-4-cyanovaleric acid of 84% purity were dissolved.

Both the solutions were added dropwise at constant rates to the reactionvessel as in Example 10, but the reaction mixture lost its flowabilityin the course of the dropwise addition and at last, it gelated. Thegelated polymer is designated as "C-16". It was impossible to measurethe various properties of "C-16" because further dilution permittedneither dispersion nor dissolution of "C-16".

COMPARATIVE EXAMPLE 17

In the reaction vessel, 330 g of purified water were charged. Whilenitrogen gas was blown into the vessel, the internal temperature wasadjusted to 80° C.

On the other hand, 397 g of a 50% aqueous solution of acrylamide, 21.4 gof N,N-dimethylaminoethyl methacrylate, 8.85 g of itaconic acid and, asa molecular weight modifier, 23.7 g of allyl alcohol were stirred into asolution. The resulting solution was adjusted to pH 4.2 with 35% HCl.

Also prepared were 60 g of an aqueous solution in which 0.9 g of4,4'-azobis-4-cyanovaleric acid of 84% purity was dissolved.

Both the solutions were added dropwise to the reaction vessel atconstant rates for 150 minutes. IN the course of the dropwise addition,the internal temperature of the reaction vessel was maintained at 80° C.After the completion of the dropwise addition, polymerization wascontinued at 80° C. for three hours. The reaction vessel was then cooledto terminate the reaction, thereby obtaining an aqueous solution of anacrylamide polymer having a Brookfield viscosity at 25° C. of 3,890 cps.The polymer so obtained is designated as "C-17". Various physicalproperties of "C-17" were measured as in Example 1.

Compositions and various properties of the polymers "A-16" to "A-18" and"C-15" to "C-17", which have been obtained in Examples 16-18 andComparative Examples 15-17, respectively are presented in Table 7.

                  TABLE 7                                                         ______________________________________                                               Examples      Comp. Ex.                                                       16    17      18      15    16    17                                   ______________________________________                                        AM (mole %)                                                                            92.5    91.1    94.8  92.4  92.9  94.0                               MBA (mole %)                                                                            0.08    0.1     0.2   0.06  0.1  0                                  SMS (mole %)                                                                            0.47    1.33    3.0  0     0     0                                  DM (mole %)                                                                            5       4       --    5     5     4                                  IA (mole %)                                                                            2        1.5    --    2     2     2                                  AA.sup.1) (mole %)                                                                     0       2       2     0     0     0                                  Initiator.sup.2)                                                                       A       A       C     D     A     A                                  (Amount   0.1     0.1     0.2   0.5   0.3   0.22                              g/M.sup.3))                                                                   DPC (%)  22.1    33.2    43.9  23.9  --    31.1                               Viscosity (ps)                                                                         89.5    120     255   461   GEL   38.9                               Mw       320     264     184   41.0  --    16.0                               (× 10,000)                                                              ______________________________________                                         .sup.1) AA: Acrylic acid                                                      .sup.2) Initiator                                                             .sup.  C: 2,2azobis-2-methylpropionamidine hydrochloride                      .sup.  D: 2,2azobis-2-(2-imidazolin-2-yl)propane hydrochloride                .sup.3) Amount of the initiator: gram number per mole of the monomer(s)       employed                                                                      Refer to Table 6 concerning other abbreviations.                         

Application Examples 2-3, Comparative Application Examples 1-2

To a 1% slurry of L-BKP having CSF of 400 ml, aluminum sulfate was addedin an amount of 0.5% based on the amount of the pulp, followed bystirring for 3 minutes. The pulp slurry at that time had pH of 6.0.Under stirring, a 1% aqueous solution of Polymer "A-16" was added in anamount of 0.5% based on the amount of the pulp on a nonvolatile basis.Stirring was continued further for 3 minutes. Using the pulp slurry soobtained, freeness was measured (JIS-P8121) and paper was made by aTAPPI square sheet machine. The wet sheet so obtained was dried at 110°C. for 3 minutes in a drum drier, whereby a hand-made paper of 100 g/m²in basis weight was obtained. The paper so dried was subjected tomoisture conditioning for at least 24 hours in an air-conditionedchamber of 20° C. and RH 65%, followed by measurement of burst index(JIS-P8112) and internal bond strength (internal bond tester,manufactured by Kumagawa Rikikogyo Co., Ltd.). Polymers "A-17", "C-15"and "C-17" were processed likewise. The results are shown in Table 8.

                  TABLE 8                                                         ______________________________________                                                     Burst index                                                                             Freeness Internal bond                                 Polymer      (kgf/cm.sup.2)                                                                          (ml)     strength (kg-cm)                              ______________________________________                                                Not added                                                                              3.31      415    3.12                                        Application                                                                           A-7      4.21      465    5.83                                        Example 2                                                                     Application                                                                           A-8      4.33      460    5.76                                        Example 3                                                                     Comparative                                                                           C-5      3.89      430    4.14                                        Application                                                                   Example 1                                                                     Comparative                                                                           C-7      3.99      440    4.23                                        Application                                                                   Example 2                                                                     ______________________________________                                    

Each aqueous solution of an acrylamide polymer according to the presentinvention is, as shown in Example 10 et seq., an aqueous polymersolution of low viscosity, although the concentration and molecularweight of the polymer are so high that no conventional art has everachieved. Such a high concentration makes it possible to save theshipment cost when considered in terms of the solid content. The polymeris therefore economically excellent. When this polymer is used as apaper strength agent, it can exhibit comparable or better performancecompared with conventional paper strength agents and is evidentlyexcellent.

EXAMPLE 19

In a four-necked 1-l separable flask equipped with a stirrer, athermometer, a reflux condenser and a nitrogen inlet tube, 330 g of 40%acrylamide, 1 g of sodium methallylsulfonate, 0.5 g of methylenebisacrylamide and 558 g of tap water were charged, followed by pHadjustment to 4.2. While the flask was purged with nitrogen, itstemperature was raised to 50° C. Ammonium persulfate was then added tothe resulting mixture and polymerization was conducted for 120 minutes.The flask was cooled to complete the polymerization reaction, therebyobtaining a stable, water-soluble polymer having a Brookfield viscosityat 25° C. of 7,400 cps and pH of 4.2. The absolute weight averagemolecular weight of the resulting polymer determined by the methoddescribed above was 2,400,000. The polymer is designated as "A".

EXAMPLE 20

In the same separable flask employed in Example 19, 330 g of 40%acrylamide, 14 g of dimethylaminoethyl methacrylate, 1 g of sodiummethallylsulfonate, 0.5 g of methylene bisacrylamide and 615 g of tapwater were charged, followed by pH adjustment to 4.2. While the flaskwas purged with nitrogen, the temperature of the resulting mixture wasraised to 50° C. Ammonium persulfate was then added to the resultingmixture and polymerization was conducted for 120 minutes. The flask wascooled to complete the polymerization reaction, thereby obtaining astable, water-soluble polymer having a Brookfield viscosity at 25° C. of7,000 cps and pH of 4.2. The absolute weight average molecular weight ofthe resulting polymer determined by the method described above was2,500,000. The polymer is designated as "B".

EXAMPLE 21

In the separable flask, 330 g of 40% acrylamide, 16 g ofdimethylaminoethyl methacrylate, 8 g of 80% acrylic acid, 3 g ofitaconic acid, 1 g of sodium methallylsulfonate, 0.5 g of methylenebisacrylamide and 669 g of tap water were charged, followed by pHadjustment to 4.2. While the flask was purged with nitrogen, thetemperature of the resulting mixture was raised to 50° C. Ammoniumpersulfate was then added to the resulting mixture and polymerizationwas conducted for 120 minutes. The flask was cooled to complete thepolymerization reaction, thereby obtaining a stable, water-solublepolymer having a Brookfield viscosity at 25° C. of 5,900 cps and pH of4.2. The absolute weight average molecular weight of the resultingpolymer determined by the method described above was 2,800,000. Thepolymer is designated as "C".

EXAMPLE 22

In the separable flask, 330 g of 40% acrylamide, 5 g ofdimethylaminoethyl methacrylate, 18 g of 80%N-metacryloyloxyethyltrimethylammonium chloride, 10 g of 80% acrylicacid, 1 g of sodium methallylsulfonate, 0.5 g of methylene bisacrylamideand 698 g of tap water were charged, followed by pH adjustment to 4.2.While the flask was purged with nitrogen, the temperature of theresulting mixture was raised to 50° C. Ammonium persulfate was thenadded to the resulting mixture and polymerization was conducted for 120minutes. The flask was cooled to complete the polymerization reaction,thereby obtaining a stable, water-soluble polymer having a Brookfieldviscosity at 25° C. of 6,200 cps and pH of 4.2. The absolute weightaverage molecular weight of the resulting polymer determined by themethod described above was 2,700,000. The polymer is designated as "D".

EXAMPLE 23

In the separable flask, 330 g of 40% acrylamide, 25 g of 80%N-metacryloyloxyethyltrimethylammonium chloride, 10 g of 80% acrylicacid, 1 g of sodium methallylsulfonate, 0.5 g of methylene bisacrylamideand 710 g of tap water were charged, followed by pH adjustment to 4.2.While the flask was purged with nitrogen, the temperature of theresulting mixture was raised to 50° C. Ammonium persulfate was thenadded to the resulting mixture and polymerization was conducted for 120minutes. The flask was cooled to complete the polymerization reaction,thereby obtaining a stable, water-soluble polymer having a Brookfieldviscosity at 25° C. of 6,800 cps and pH of 4.2. The absolute weightaverage molecular weight of the resulting polymer determined by themethod described above was 2,600,000. The polymer is designated as "E".

EXAMPLE 24

In the separable flask, 330 g of 40% acrylamide, 20 g of 80% acrylicacid, 1 g of sodium methallylsulfonate, 0.5 g of methylene bisacrylamideand 645 g of tap water were charged, followed by pH adjustment to 4.2.While the flask was purged with nitrogen, the temperature of theresulting mixture was raised to 50° C. Ammonium persulfate was thenadded to the resulting mixture and polymerization was conducted for 120minutes. The flask was cooled to complete the polymerization reaction,thereby obtaining a stable, water-soluble polymer having a Brookfieldviscosity at 25° C. of 5,400 cps and pH of 4.2. The absolute weightaverage molecular weight of the resulting polymer determined by themethod described above was 2,500,000. The polymer is designated as "F".

EXAMPLE 25

In the separable flask, 330 g of 40% acrylamide, 12 g of 80% acrylicacid, 5 g of itaconic acid, 1 g of sodium methallylsulfonate, 0.5 g ofmethylene bisacrylamide and 615 g of tap water were charged, followed bypH adjustment to 4.2. While the flask was purged with nitrogen, thetemperature of the resulting mixture was raised to 50° C. Ammoniumpersulfate was then added to the resulting mixture and polymerizationwas conducted for 120 minutes. The flask was cooled to complete thepolymerization reaction, thereby obtaining a stable, water-solublepolymer having a Brookfield viscosity at 25° C. of 61,000 cps and pH of4.2. The absolute weight average molecular weight of the resultingpolymer determined by the method described above was 2,400,000. Thepolymer is designated as "G".

COMPARATIVE EXAMPLE 18

In the separable flask, 330 g of 40% acrylamide, 1 g of sodiummethallylsulfonate and 555 g of tap water were charged, followed by pHadjustment to 4.2. While the flask was purged with nitrogen, thetemperature of the resulting mixture was raised to 50° C. Ammoniumpersulfate was then added to the resulting mixture and polymerizationwas conducted for 120 minutes. The flask was cooled to complete thepolymerization reaction, thereby obtaining a stable, water-solublepolymer having a Brookfield viscosity at 25° C. of 8,300 cps and pH of4.2. The absolute weight average molecular weight of the resultingpolymer determined by the method described above was 700,000. Thepolymer is designated as "H".

COMPARATIVE EXAMPLE 19

In the separable flask, 330 g of 40% acrylamide, 0.5 g of methylenebisacrylamide and 553 g of tap water were charged, followed by pHadjustment to 4.2. While the flask was purged with nitrogen, itstemperature was raised to 50° C. Ammonium persulfate was then added tothe resulting mixture and polymerization was conducted for 120 minutes.The flask was cooled to complete the polymerization reaction, therebyobtaining a stable, water-soluble polymer having a Brookfield viscosityat 25° C. of 5,400 cps and pH of 4.2. The absolute weight averagemolecular weight of the resulting polymer determined by the methoddescribed above was 900,000. The polymer is designated as "I".

COMPARATIVE EXAMPLE 20

In the separable flask, 330 g of 40% acrylamide, 14 g ofdimethylaminoethyl methacrylate, 1 g of sodium methallylsulfonate and607 g of tap water were charged, followed by pH adjustment to 4.2. Whilethe flask was purged with nitrogen, the temperature of the resultingmixture was raised to 50° C. Ammonium persulfate was then added to theresulting mixture and polymerization was conducted for 120 minutes. Theflask was cooled to complete the polymerization reaction, therebyobtaining a stable, water-soluble polymer having a Brookfield viscosityat 25° C. of 8,000 cps and pH of 4.2. The absolute weight averagemolecular weight of the resulting polymer determined by the methoddescribed above was 970,000. The polymer is designated as "J".

COMPARATIVE EXAMPLE 21

In the separable flask, 330 g of 40% acrylamide, 16 g ofdimethylaminoethyl methacrylate, 8 g of 80% acrylic acid, 3 g ofitaconic acid, 1 g of sodium methallylsulfonate and 659 g of tap waterwere charged, followed by pH adjustment to 4.2. While the flask waspurged with nitrogen, the temperature of the resulting mixture wasraised to 50° C. Ammonium persulfate was then added to the resultingmixture and polymerization was conducted for 120 minutes. The flask wascooled to complete the polymerization reaction, thereby obtaining astable, water-soluble polymer having a Brookfield viscosity at 25° C. of6,700 cps and pH of 4.2. The absolute weight average molecular weight ofthe resulting polymer determined by the method described above was810,000. The polymer is designated as "K".

COMPARATIVE EXAMPLE 22

In the separable flask, 330 g of 40% acrylamide, 5 g ofdimethylaminoethyl methacrylate, 18 g of 80%N-metacryloyloxyethyltrimethylammonium chloride, 10 g of 80% acrylicacid, 1 g of sodium methallylsulfonate and 683 g of tap water werecharged, followed by pH adjustment to 4.2. While the flask was purgedwith nitrogen, the temperature of the resulting mixture was raised to50° C. Ammonium persulfate was then added to the resulting mixture andpolymerization was conducted for 120 minutes. The flask was cooled tocomplete the polymerization reaction, thereby obtaining a stable,water-soluble polymer having a Brookfield viscosity at 25° C. of 6,000cps and pH of 4.2. The absolute weight average molecular weight of theresulting polymer determined by the method described above was1,000,000. The polymer is designated as "L".

COMPARATIVE EXAMPLE 23

In the separable flask, 330 g of 40% acrylamide, 25 g of 80%N-metacryloyloxyethyltrimethylammonium chloride, 10 g of 80% acrylicacid, 1 g of sodium methallylsulfonate and 707 g of tap water werecharged, followed by pH adjustment to 4.2. While the flask was purgedwith nitrogen, the temperature of the resulting mixture was raised to50° C. Ammonium persulfate was then added to the resulting mixture andpolymerization was conducted for 120 minutes. The flask was cooled tocomplete the polymerization reaction, thereby obtaining a stable,water-soluble polymer having a Brookfield viscosity at 25° C. of 7,100cps and pH of 4.2. The absolute weight average molecular weight of theresulting polymer determined by the method described above was1,100,000. The polymer is designated as "M".

COMPARATIVE EXAMPLE 24

In the separable flask, 330 g of 40% acrylamide, 20 g of 80% acrylicacid, 1 g of sodium methallylsulfonate and 642 g of tap water werecharged, followed by pH adjustment to 4.2. While the flask was purgedwith nitrogen, the temperature of the resulting mixture was raised to50° C. Ammonium persulfate was then added to the resulting mixture andpolymerization was conducted for 120 minutes. The flask was cooled tocomplete the polymerization reaction, thereby obtaining a stable,water-soluble polymer having a Brookfield viscosity at 25° C. of 6,900cps and pH of 4.2. The absolute weight average molecular weight of theresulting polymer determined by the method described above was 890,000.The polymer is designated as "N".

EXAMPLE 26

In the separable flask, 330 g of 40% acrylamide, 12 g of 80% acrylicacid, 5 g of itaconic acid, 1 g of sodium methallylsulfonate and 638 gof tap water were charged, followed by pH adjustment to 4.2. While theflask was purged with nitrogen, the temperature of the resulting mixturewas raised to 50° C. Ammonium persulfate was then added to the resultingmixture and polymerization was conducted for 120 minutes. The flask wascooled to complete the polymerization reaction, thereby obtaining astable, water-soluble polymer having a Brookfield viscosity at 25° C. of73,000 cps and pH of 4.2. The absolute weight average molecular weightof the resulting polymer determined by the method described above was880,000. The polymer is designated as "O". Application Examples 4-10,Comparative Application Examples 3-10

To a 1% pulp slurry which was obtained from waste corrugated fiberboardsand had a CSF of 420 ml, aluminum sulfate was added in an amount of 1.0%based on the amount of the pulp on a dry weight basis, followed bystirring for one minute. The pulp slurry at that time had a pH of 5.0.Incidentally, pulp slurries prepared in a similar manner except thatsodium hydroxide was added in varied amounts immediately before theaddition of aluminum sulfate had pHs of 5.0, 6.0 and 7.0. The paperstrength agent obtained in Example 19 was added in an amount of 0.5%based on the amount of the pulp on a dry weight basis, followed bystirring for further one minute. Using a TAPPI square sheet machine, theslurry was machined into a paper. The wet sheet so machined was dried at110° C. for 3 minutes in a drum drier, whereby a hand-made paper of 150g/m² in basis weight was obtained as Application Example 4. The paper sodried was subjected to moisture conditioning for at least 24 hours in anair-conditioned chamber of 20° C. and RH 65%, followed by measurement ofburst index (JIS-P8112), ring crush (JIS-P8126) and freeness(JIS-P8121). The results are shown in Tables 8-10.

In Application Examples 5-10 and Comparative Application Examples 3-10,papers were formed under similar conditions in a similar manner exceptthat the paper strength agent (A) was replaced by those indicated in thetable shown below, respectively. Further, their burst index, ring crushand freeness were measured and evaluated also under similar conditionsand in a similar manner to Application Example 4.

                  TABLE 9                                                         ______________________________________                                                            (machined at pH 5.0)                                             Polymer                                                                              Burst index                                                                              Ring crush                                                                             Freeness                                           added  (kgf/cm.sup.2)                                                                           (N · m.sup.2 /g)                                                              (ml)                                        ______________________________________                                                 Not added                                                                              3.52       15.3   420                                       Application                                                                            A        4.72       19.4   445                                       Example 4                                                                     Application                                                                            B        4.81       21.0   465                                       Example 5                                                                     Application                                                                            C        4.85       21.4   465                                       Example 6                                                                     Application                                                                            D        4.86       21.0   470                                       Example 7                                                                     Application                                                                            E        4.80       20.6   480                                       Example 8                                                                     Application                                                                            F        4.87       21.6   480                                       Example 9                                                                     Application                                                                            G        4.90       21.6   475                                       Example 10                                                                    Comparative                                                                            H        4.22       18.1   430                                       Application                                                                   Example 3                                                                     Comparative                                                                            I        4.24       18.3   440                                       Application                                                                   Example 4                                                                     Comparative                                                                            J        4.38       18.5   440                                       Application                                                                   Example 5                                                                     Comparative                                                                            K        4.20       18.2   440                                       Application                                                                   Example 6                                                                     Comparative                                                                            L        4.21       18.0   430                                       Application                                                                   Example 7                                                                     Comparative                                                                            M        4.25       18.6   420                                       Application                                                                   Example 8                                                                     Comparative                                                                            N        4.30       19.0   440                                       Application                                                                   Example 9                                                                     Comparative                                                                            O        4.31       19.1   435                                       Application                                                                   Example 10                                                                    ______________________________________                                    

                  TABLE 10                                                        ______________________________________                                                            (Machined at pH 6.0)                                             Polymer                                                                              Burst index                                                                              Ring crush                                                                             Freeness                                           added  (kgf/cm.sup.2)                                                                           (N · m.sup.2 /g)                                                              (ml)                                        ______________________________________                                                 Not added                                                                              3.65       16.0   425                                       Application                                                                            A        4.70       19.5   450                                       Example 4                                                                     Application                                                                            B        4.85       21.3   465                                       Example 5                                                                     Application                                                                            C        4.91       21.8   460                                       Example 6                                                                     Application                                                                            D        4.90       21.5   465                                       Exarnple 7                                                                    Application                                                                            E        4.88       21.0   475                                       Example 8                                                                     Application                                                                            F        4.80       20.2   470                                       Example 9                                                                     Application                                                                            G        4.84       21.0   465                                       Example 10                                                                    Comparative                                                                            H        4.25       18.2   420                                       Application                                                                   Example 3                                                                     Comparative                                                                            I        4.26       18.3   430                                       Application                                                                   Example 4                                                                     Comparative                                                                            J        4.43       19.0   440                                       Application                                                                   Example 5                                                                     Comparative                                                                            K        4.25       18.9   430                                       Application                                                                   Example 6                                                                     Comparative                                                                            L        4.22       18.5   425                                       Application                                                                   Example 7                                                                     Comparative                                                                            M        4.25       19.0   425                                       Application                                                                   Example 8                                                                     Comparative                                                                            N        4.26       18.6   450                                       Application                                                                   Example 9                                                                     Comparative                                                                            O        4.28       19.4   445                                       Application                                                                   Example 10                                                                    ______________________________________                                    

                  TABLE 11                                                        ______________________________________                                                            (Machined at pH 7.0)                                             Polymer                                                                              Burst index                                                                              Ring crush                                                                             Freeness                                           added  (kgf/cm.sup.2)                                                                           (N · m.sup.2 /g)                                                              (ml)                                        ______________________________________                                                 Not added                                                                              3.55       15.1   420                                       Application                                                                            A        4.64       19.1   445                                       Example 4                                                                     Application                                                                            B        4.81       21.0   455                                       Example 5                                                                     Application                                                                            C        4.88       21.3   460                                       Example 6                                                                     Application                                                                            D        4.90       21.6   460                                       Example 7                                                                     Application                                                                            E        4.82       20.6   460                                       Example 8                                                                     Application                                                                            F        4.76       20.0   450                                       Example 9                                                                     Application                                                                            G        4.81       20.7   455                                       Example 10                                                                    Comparative                                                                            H        3.92       17.2   425                                       Application                                                                   Example 3                                                                     Comparative                                                                            I        3.99       17.9   430                                       Application                                                                   Example 4                                                                     Comparative                                                                            J        4.20       18.9   425                                       Application                                                                   Example 5                                                                     Comparative                                                                            K        4.22       19.4   430                                       Application                                                                   Example 6                                                                     Comparative                                                                            L        4.15       18.0   430                                       Application                                                                   Example 7                                                                     Comparative                                                                            M        4.23       18.7   430                                       Application                                                                   Example 8                                                                     Comparative                                                                            N        3.89       17.6   440                                       Application                                                                   Example 9                                                                     Comparative                                                                            O        3.98       18.4   435                                       Application                                                                   Example 10                                                                    ______________________________________                                    

As is apparent from the results shown in Tables 8-10, each paperstrength agent according to the present invention is superior to theconventional ones, because it shows excellent paper strength reinforcingeffects in terms of burst index, ring crush and freeness and isresistant to pH variations of a pulp slurry to be formed into paper.

What is claimed is:
 1. An aqueous solution of an acrylamide polymer,which has a polymer concentration ranging from 22 to 60%, a Brookfieldviscosity at 25° C. of 50,000 cps or smaller and a weight-averagemolecular weight of from 500,000 to 10,000,000, wherein said polymer isobtained by polymerizing acrylamide, methacrylamide, or one or morevinyl monomers in addition to acrylamide and/or methacrylamide with oneor more of vinyl compounds and/or salts thereof, said compounds beingrepresented by the following formula (I): ##STR3## wherein R representsa hydrogen atom or a lower alkyl group and n stands for an integer offrom 1 to 8, and also comprising one or more crosslinking monomers as acopolymer component or copolymer components.
 2. A paper strength agentcomprising an acrylamide polymer having:(a) a weight-average molecularweight of from 1,500,000 to 10,000,000; (b) a weight-average root meansquare radius of from 30 to 150 nm; and (c) a number-average molecularweight of 400,000 to 5,000,000; wherein the ratio (a)/(c) of theweight-average molecular weight to the number-average molecular weightis 6 or smaller and the ratio (b)/(a) of the weight-average root meansquare radius to the weight-average molecular weight is 0.00004 orsmaller.
 3. A paper strength agent obtained by polymerizing thefollowing components:(a) from 94 to 99.98 mol % of (meth)acrylamide; (b)from 0.01 to 1 mol % of one or more of crosslinking monomers; and (c)from 0.01 to 5 mol % of one or more of vinyl compounds and/or saltsthereof, said vinyl compounds being represented by the following formula(2): ##STR4## wherein R₁ represents a hydrogen atom or a C₁₋₃ loweralkyl group and n stands for an integer of from 1 to
 4. 4. A paperstrength agent obtained by polymerizing the following components:(a)from 50 to 99.97 mol % of (meth)acrylamide; (b) from 0.01 to 1 mol % ofone or more of crosslinking monomers; and (c) from 0.01 to 5 mol % ofone or more of vinyl compounds and/or salts thereof, said vinylcompounds being represented by the following formula (2): ##STR5##wherein R₁ represents a hydrogen atom or a C₁₋₃ lower alkyl group and nstands for an integer of from 1 to 4; and (d) 0.01-49.98 mol % of one ormore vinyl compounds and/or salts thereof copolymerizable with saidmonomers.
 5. An aqueous solution of an acrylamide polymer, which has apolymer concentration ranging from 22 to 60%, a Brookfield viscosity at25° C. of 50,000 cps or smaller and a weight-average molecular weight offrom 500,000 to 10,000,000.
 6. An aqueous solution according to claim 5,obtained by polymerizing acrylamide, methacrylamide, or one or morevinyl monomers in addition to acrylamide and/or methacrylamide with oneor more of vinyl compounds and/or salts thereof, said compounds beingrepresented by the following formula (I): ##STR6## wherein R representsa hydrogen atom or a lower alkyl group and n stands for an integer offrom 1 to
 8. 7. An aqueous solution according to claim 1, obtained by asemi-batchwise polymerization wherein the monomer(s) are added dropwise.8. A paper strength agent comprising an acrylamide polymer having:(a) aweight-average molecular weight of from 1,500,000 to 10,000,000 and (b)a weight-average root mean square radius of from 30 to 150 nm, (c) theratio (b)/(a) of the weight-average root mean square radius to theweight-average molecular weight being 0.00004 or smaller.
 9. A paperstrength agent comprising an acrylamide polymer according to claim 2, atleast 40 wt. % of which comprises a polymer having a molecular weight of1,000,000 or greater.
 10. A paper strength agent comprising anacrylamide polymer according to claim 8, having a Brookfield viscosityat 25° C. of from 20 to 10,000 cps in the form of a 10 wt. % aqueoussolution or a Brookfield viscosity at 25° C. of from 100 to 30,000 cpsin the form of a 15 wt. % aqueous solution.
 11. A paper strength agentcomprising an acrylamide polymer according to Clam 10, having a hydrateddiameter of from 50 to 300 nm as measured by a dynamic light scatteringmethod.
 12. A paper strength agent obtained by polymerizing acrylamide,methacrylamide, or one or more of monomers in addition to acrylamideand/or methacrylamide with one or more of compounds and/or saltsthereof, said compounds being represented by the following formula (I):##STR7## wherein R represents a hydrogen atom or a lower alkyl group andn stands for an integer of from 1 to 8, and which comprises one or moreof crosslinking monomers as a copolymer component or copolymercomponents.
 13. A paper strength agent according to claim 12, whichcomprises an acrylamide polymer having:(a) a weight-average molecularweight of from 1,500,000 to 10,000,000 and (b) a weight-average rootmean square radius of from 30 to 150 nm, (c) the ratio (b)/(a) of theweight-average root mean square radius to the weight-average molecularweight being 0.00004 or smaller.
 14. A paper strength agent comprisingan aqueous solution of an acrylamide polymer, which has a polymerconcentration ranging from 22 to 60%, a Brookfield viscosity at 25° C.of 50,000 cps or smaller and a weight-average molecular weight of from500,000 to 10,000,000.
 15. A paper strength agent comprising an aqueoussolution of an acrylamide polymer according to claim 14, obtained bypolymerizing acrylamide, methacrylamide, or one or more vinyl monomersin addition to acrylamide and/or methacrylamide with one or more ofvinyl compounds and/or salts thereof, said compounds being representedby the following formula (1): ##STR8## wherein R represents a hydrogenatom or a lower alkyl group and n stands for an integer of from 1 to 8.16. A paper strength agent comprising an aqueous solution of anacrylamide polymer according to claim 15, which comprises one or more ofcrosslinking monomers as a copolymer component or copolymer components.17. A paper strength agent comprising an aqueous solution of anacrylamide polymer according to claim 16, obtained by a semi-batchwisepolymerization wherein the monomer(s) are added dropwise.
 18. A paperstrength agent comprising an aqueous solution of an acrylamide polymeraccording to claim 15, which comprises at lease one or more of cationicmonomers and anionic monomers as a copolymer component or copolymercomponents.
 19. A paper strength agent comprising an aqueous solution ofan acrylamide polymer according to claim 16, which comprises at leastone or more of cationic monomers and anionic monomers as a copolymercomponent or copolymer components.
 20. Paper comprising a paper strengthagent according to claim 8.